A common developer situation is that you have a project with a bunch of dependencies. You are working on the dependencies and want to add a dependency to one of them.

As a concrete example, let's say I have:

(Env) pkg> st
    Status `Project.toml`
  [7876af07] Example v0.5.1+ [`~/.julia/dev/Example`]
  [682c06a0] JSON v0.18.0+ [`~/.julia/dev/JSON`]

Now, I want to use Example in JSON.

The current steps to properly do this before JSON can be loaded is:

1. Add import Example to JSON.
2. Make the JSON project your active project.
3. Do Pkg.add("Example") to update Project.toml in JSON (this could potentially fail due to some weird constraints in the Manifest file in the JSON project)
4. Make your previous main project your active project
5. Do Pkg.resolve() to update the main project's Manifest.toml, which looks at JSON's Project.toml (populate deps = ["Example"] for JSON).
6. Load JSON.

Arguably, the steps 2-5 is kinda annoying while you are in the heat of developing. The reason we need to lookup the deps section in the Manifest is because `JSON´ could potentially have used another package (another UUID) that also had the name Example. However, sometimes the Project.toml Example is the only package with that name in the Manifest, and it is likely that that is the package that one wanted to load into JSON anyway. So it feels a bit cruel to not allow it to just be loaded.

This PR instead makes the workflow:

1. Add import Example to JSON.
2. Load JSON.

with the result

julia> import JSON
[ Info: Precompiling module JSON
┌ Warning: Package JSON does not have Example in its dependencies:
│  - If you have JSON checked out for development and have
│    added Example as a dependency but haven't updated your primary
│    environment's manifest file, try `Pkg.resolve()`.
│  - Otherwise you may need to report an issue with JSON
│
│ Attempting fallback to project dependency with name Example.
└ @ Base loading.jl:817

julia>

So it is quite clear that something wrong is happening but you can still continue working.

There are of course other solutions to this issue:

• Make steps 2-5 easier (pkg> add JSON:Example; resolve)
• Somehow detect when this situation occurs and do it manually.

The current implementation is not perfect. It will look for fallback packages in environments below you in the stack which is perhaps not wanted. Also, maybe this should only be enabled for packages that are in "developer mode", i.e. tracking a path.

cc @Keno, @stevengj who has been "bitten" by this issue, cc @StefanKarpinski