…ank you!)

Simple oversight that deep variables can reference overwrites too.  This fix acknowledges this oversight by using the standard logic for using overwrite instead of getValueFromSource in this case.

…(DV) cases

Solve for these cases.  The bug was that DVs can render to DVs when a value is being obtained from them (i.e. they are being de-referenced in getValueFromDeep).  This is particularly problematic in the case that the original DV being rendered has a deep reference into the rendered DV.  In that case the DV returned by rendering must be replaced by yet another DV.

While accomplishing the above, various bits of cleanup were implemented and I added some commentary around getDeeperValue for future engineers.

This case is particularly relevant if you have:

`serverless.yml`:
```
service: serverless-hello-world

provider:
  name: aws
  runtime: nodejs6.10
  stage: dev
  environment:
    SECRET: ${self:custom.secrets.SECRET}

functions:
  helloWorld:
    handler: handler.helloWorld
    events:
      - http:
          path: hello-world
          method: get
          cors: true

custom:
  stage: ${opt:stage, self:provider.stage}
  secrets: ${file(secrets.${self:custom.stage}.yml)}
```
AND
`secrets.dev.yml`:
```
SECRETS: secrets
```

Populating this service should result in the following sequence of rendering phases:

#########################
# PHASE 1
#
# ${self:custom.secrets.SECRET}
#   <- ${deep:0.SECRET}
#   deep[0] = ${file(secrets.${self:custom.stage}.yml)}
#
# ${opt:stage, self:provider.stage}
#   <- 'dev'
#
# ${file(secrets.${self:custom.stage}.yml)}
#   <- ${file(secrets.${deep:1}.yml)}
#   deep[1] = ${opt:stage, self:provider.stage}
#
# RESULT
#
# service: serverless-hello-world
#
# provider:
#   name: aws
#   runtime: nodejs6.10
#   stage: dev
#   environment:
#     SECRET: ${deep:0.SECRET}
#
# functions:
#   helloWorld:
#     handler: handler.helloWorld
#     events:
#       - http:
#           path: hello-world
#           method: get
#           cors: true
#
# custom:
#   stage: dev
#   secrets: ${file(secrets.${deep:1}.yml)}
#########################

#########################
# PHASE 2
#
# ${deep:0.SECRET}
#   <- this.populateValue('${file(secrets.${self:custom.stage}.yml)}') => '${file(secrets.${deep:1}.yml)}' => '${deep:2}' => '${deep:2.SECRET}'
#   deep[2] = ${file(secrets.${deep:1}.yml)}
#
# ${file(secrets.${deep:1}.yml)}
#   <- this.populateValue('${file(secrets.${deep:1}.yml)}') => '${file(secrets.dev.yml)}' => '${deep:3}'
#   deep[3] = ${file(secrets.dev.yml)}
#
# RESULT
#
# service: serverless-hello-world
#
# provider:
#   name: aws
#   runtime: nodejs6.10
#   stage: dev
#   environment:
#     SECRET: ${deep:2.SECRET}
#
# functions:
#   helloWorld:
#     handler: handler.helloWorld
#     events:
#       - http:
#           path: hello-world
#           method: get
#           cors: true
#
# custom:
#   stage: dev
#   secrets: ${deep:3}
#########################

#########################
# PHASE 3
#
# ${deep:2.SECRET}
#   <- this.populateValue('${file(secrets.${deep:1}.yml)}') => '${file(secrets.dev.yml)}' => '${deep:3}' => '${deep:3.SECRET}'
#   deep[3] is already set to ${file(secrets.dev.yml)}
#
# ${deep:3}
#   <- this.populateValue('${file(secrets.dev.yml)}') => { SECRET: 'secret' }
#
# RESULT
#
# service: serverless-hello-world
#
# provider:
#   name: aws
#   runtime: nodejs6.10
#   stage: dev
#   environment:
#     SECRET: ${deep:3.SECRET}
#
# functions:
#   helloWorld:
#     handler: handler.helloWorld
#     events:
#       - http:
#           path: hello-world
#           method: get
#           cors: true
#
# custom:
#   stage: dev
#   secrets:
#     SECRET: secret
#########################

#########################
# PHASE 4
#
# ${deep:3}
#   <- this.populateValue('${file(secrets.dev.yml)}') => this.getDeeperValue(['SECRET'], { SECRET: 'secret' }) => 'secret'
#
# RESULT
#
# service: serverless-hello-world
#
# provider:
#   name: aws
#   runtime: nodejs6.10
#   stage: dev
#   environment:
#     SECRET: secret
#
# functions:
#   helloWorld:
#     handler: handler.helloWorld
#     events:
#       - http:
#           path: hello-world
#           method: get
#           cors: true
#
# custom:
#   stage: dev
#   secrets:
#     SECRET: secret
#########################

Fix `print`
The print command is highly linked to the `Variables` and `Service` codebases, keep those in sync and leave reminders about the link.  Made these explicit and separately implemented to avoid complexity.
Additionally, the print command re-populates an object with the *very similar* content as the previously pre-populated service (just not augmented as just mentioned).  This can lead to cross contamination between the two.  As such, all caches must be cleared per unique invocation of service/object/property population.
Add tests for some expected but previously unverified behaviors.

Clean pre-population
The previous implementation worked okay but was unnecessary and would have been a maintenance problem.  Instead, just knock out the population of variables depending on those config dependent services and use the standard means of resolution.

Fix cyclic bug (resulting from running print against a self-referencing serverless.yml)
The caching of values could lead to a cyclic object remaining in the caches for variable population.  This causes crashes and pain.  Solved by the cache cleaning logic.

The framework's Service class modifies a user's service, mutating a given `service: { name: 'string' }` to `service: 'string'` but doesn't account for this in the variables system.  This is the basis for serverless#4744.

While working this area, I discovered that the Service class also does this to the provider, accepting `provider: 'aws'` and replacing it with `provider: { name: 'aws' }`, causing the natural `${self:provider}` to fail.

Catching either 'self:service.name' or 'self:provider' and replacing them with the mutated reference solves this user confusion.