dependencies: handle one more case of subproject installed files

Some projects treat meson.project_source_root() as the root of the
dependency files, because the project itself merely wraps a bunch of
datafiles. Our validation to make sure this doesn't point to another
subproject, made use of pathlib.Path's generator for all component
paths, which... did not include the path itself. So go ahead and
explicitly check that too. Add a test case to verify it while we are at
it.

Fixes https://github.com/mesonbuild/meson/pull/10103#issuecomment-1114901033

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