nimbus-gradle-plugin: Support absolute and relative paths.

[linabutler]

Hi @jeddai! 👋🏼

When I was fixing the Fenix auto-publishing workflow last week, I noticed that it didn't like absolute paths to the Application Services repo. I added a low-effort check that threw an exception for (Unix-style) absolute paths, but wanted to fix the issue for real.

I tracked down the bellyaching to how the Nimbus Gradle Plugin was resolving paths—and fixed it up in this PR so that it can handle both kinds of paths ☺️ I've tested this logic locally, and it works with both absolute and relative paths in autoPublish.application-services.dir now!

I think this is an O(n) change.

Here's an explanation of what I did, copied from the commit message:

• Use project.file(...) to resolve paths relative to the
  project directory, instead of the File constructor. The
  .file() method understands absolute paths.
• Use project.layout.buildDirectory.dir(...) to resolve
  paths relative to the build directory, instead of manually
  building the path from the deprecated project.buildDir
  property.
• Use / instead of File.separator for paths. This is the style
  that Gradle recommends, and it simplifies how we build path strings.
  The Java File APIs canonicalize / to \ on Windows, so it's
  not necessary to use File.separator explicitly.

And, in a follow-up commit, I noticed a couple of places where we could use the Groovy withReader method.

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
  • Note:
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    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due diligence applied in selecting them.

Branch builds: add [firefox-android: branch-name] to the PR title.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.83%. Comparing base (c6550f7) to head (8b95708).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6224   +/-   ##
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  Coverage   83.83%   83.83%           
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  Files         117      117           
  Lines       15693    15693           
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  Hits        13157    13157           
  Misses       2536     2536           

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[jeddai]

This looks great!