Remove importmap entries when uninstalling a package

[Amoifr]

Fix #1009

When AssetMapper is used, Flex runs importmap:require for the entries declared by a UX package on install, but nothing removed them on uninstall: the entries stayed in importmap.php forever.

The tricky part is that by the time PackageJsonSynchronizer runs (POST_UPDATE_CMD), the uninstalled package's files are gone, so its symfony.importmap config cannot be read anymore. And blindly diffing importmap.php against the entries declared by the remaining packages is not an option either: it would also remove the entries the user added manually with importmap:require.

So this PR captures the entry names on PRE_PACKAGE_UNINSTALL, while the package files still exist, and passes them to the synchronizer, which then runs a single importmap:remove for the captured names, except those that are:

• still declared by an installed package (e.g. two bundles sharing a JS dependency), or
• not present in importmap.php (e.g. already removed by hand).

The synchronize() signature change is backward compatible (new optional parameter), and the webpack/package.json path is untouched (obsolete file: links were already cleaned up by removeObsoletePackageJsonLinks()).

[Kocal]

What if an other UX package require the same dependency from the package you are removing... does the dependency stay in importmap.php?

[Amoifr]

Good catch to check! Yes, it stays: the removal list is filtered against the entries still declared by the installed packages, so anything another package still needs is skipped. This exact scenario is covered by testSynchronizeAssetMapperRemovesObsoleteImportMapEntries (@hotcake/foo belongs to the removed package but is still declared by symfony/new-package, and it survives the uninstall). 😄