Consolidates the code to find all changed file paths into the `PackageLoopingCommand` class that is the base of almost all of the repo tooling commands. This in a preparatory PR for a future change to allow each command to define a list of files or file patterns that definitively *don't* affect that test, so that CI can be smarter about what tests to run (e.g., not running expensive integration tests for README changes).
A side effect of this change is that tests of almost all commands now need a mock `GitDir` instance. This would add a lot of copy/pasted boilerplate to the test setup, and there is already too much of that, so instead this refactors common test setup:
- Creating a memory file system
- Populating it with a packages directory
- Creating a RecordingProcessRunner to mock out process calls
- Creating a mock GitDir that forwards to a RecordingProcessRunner
into a helper method (using records and destructuring to easily return multiple values). While some tests don't need all of these steps, those that don't can easily ignore parts of it, and it will make it much easier to update tests in the future if they need them, and it makes the setup much more consistent which makes it easier to reason about test setup in general.
Prep for https://github.com/flutter/flutter/issues/136394
Adds a new tool command (and runs it in CI) to check that each package:
- is listed correctly in the repo-level README.md table
- has a CODEOWNERS entry
In the future we could add other things (e.g., auto-label), but these were the main things we've had issues with recently.
Updates README.md and CODEOWNERS to fix failures it found:
- Adds a couple of missing CODEOWNERS
- Expands the web implementation CODEOWNERS to individual packages so that we don't have to special-case handling in the tool
- Fixes some minor mistakes in README.md
- URL-encodes all `:`s in the README.md links (which is why ever line shows as changed); it worked without that in practice, but it should really be encoded, and having it consistently encoded made things easier for the tooling.
