Remove active composer fixture files in UpdaterFormTest

worked with @kunal.sachdev on this. Functional tests are more complicated because you have to get the stage directory after the update has been stages so you can use \Drupal\Tests\package_manager\Traits\FixtureUtilityTrait::modifyPackage on the stage directory.

Since we don't know the stage id we can't claim the stage to get the directory.

We will probably need a helper function like getSTageDirectory for functional test or we could make a wrapper around modifyPackage like modifyStagedPackage that would take care of this for us

#6:

1. What is the stack trace for it?
2. What are the contents of the array?

this is a related problem we have to fix first #3320487: fake-site fixtures has invalid packages in composer files

1 review, needs input from @tedbow.

For MR comments,

Green and the last 3 commits look clean … I can't review this quite yet, so assigning to @tedbow 😊

@kunal.sachdev getting close. thanks

Also we still have a follow-up issue that we need to make from the summary, I think. If it already exists lets add it as a related issue and remove the "Needs follow-up" tag

@kunal.sachdev thanks for creating #3321904: Remove automatic_updates_extensions/tests/fixtures/stage_composer in favor of using StageFixtureManipulator

I think this looks good. I am glad the test changes are pretty small but it still lets us get rid of a bunch of fixture files. I think this speak the value of the `addPackage`, `modifyPackage` and `removePackage` methods 🎉.

Once we do #3321236: Add actual project folders in \Drupal\Tests\package_manager\Traits\FixtureUtilityTrait::addPackage I think we would also be able remove the fixture folders such as automatic_updates_extensions/tests/fixtures/two_projects/web/projects/aaa_automatic_updates_test

Indeed, +31 -161 is a nice diff stat 👏