…ages (#78)

Motivation
----------

A target in `Package.swift` can include
[resources](https://developer.apple.com/documentation/packagedescription/target/resources).
When the target is built, the resources are copied into a bundle
directory and a
[`Bundle`](https://developer.apple.com/documentation/foundation/bundle)
class is generated, giving the executable a convenient way to retrieve
them at run time.

A target with resources should be able to access them when it is
packaged in a container image. For example, a web server might bundle
resources such as images or fonts.

Fixes #48 

Modifications
-------------

* Add a new `--resource` parameter to `containertool`. This specifies a
resource bundle directory which is added to the container image using
the directory archiving support added to the `Tar` module in #74. When
`containertool` is used alone, multiple `--resource` flags can be
specified and will be added to the image in the order in which they
appear on the command line. A target in `Package.swift` can only define
one resource bundle, so when used through the plugin only one bundle
will be added.
* Update the plugin to pass the `--resource` flag to `containertool`
when an executable target has resources.

Result
------

If a target includes resources, the resource bundle will be copied into
the container image at a path where the generated `Bundle` class can
find it.
If a target does not include resources, the container image will only
include the executable.

Test Plan
---------

* Existing tests continue to pass.
* New integration tests verify that all expected resources are present.