Packages are the primary conceptual object in the tool, but currently they are represented simply as Directory (or occasionally a path string). This introduces an object for packages and:
- moves a number of existing utility methods into it
- sweeps the code for the obvious cases of using `Directory` to represent a package, especially in method signatures and migrates them
- notes a few places where we should migrate later, to avoid ballooning the size of the PR
There are no doubt other cases not caught in the sweep, but this gives us a foundation both for new code, and to migrate incrementally toward as we find existing code that was missed.
Adds a new `lint-android` command to run `gradlew lint` on Android plugins.
Also standardizes the names of the Cirrus tasks that run all the build and platform-specific (i.e., not Dart unit test) tests for each platform, as they were getting unnecessarily long and complex in some cases.
Fixes https://github.com/flutter/flutter/issues/87071
