test(plugin): compare a composer/installers run against upstream Composer

The fixture project pins composer/installers 2.3.0 and requires two packages
of framework-specific types, so the plugin's LibraryInstaller subclass decides
where they land. Upstream Composer and Shirabe install the same project and
the whole resulting tree is compared.

The plugin tarball is fetched by `fixtures/e2e-installers/fetch` into a
git-ignored directory and pinned by a digest over the extracted files, so the
third-party source never enters this repository and the test skips itself
while the directory is absent. The tree is staged and only moved into place
once verified, so an unverified tree is never observable under the name the
test looks for.

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