When collecting Poetry projects for caching, a '**/poetry.lock' glob is
used.  However, in order to process the Poetry configuration, the
"poetry" command is run from the repo's root directory; this causes
Poetry to return an invalid configuration when there is a Poetry project
inside an inner directory.

Instead of running a single Poetry command, glob for the same pattern,
and run a Poetry command for every discovered project.

When the default dependency path is used for cache distributors, they
are looking for the dependency file in the project's root (including the
source code), which leads to tests taking a significant amount of time,
especially on Windows runners.  We thus hit sporadic test failures.

Change the test cases such that dependency files are always searched for
inside of `__tests__/data`, ignoring the rest of the project.

The virtualenv cache might contain invalid entries, such as virtualenvs
built in previous, buggy versions of this action.  The `poetry env use`
command will recreate virtualenvs in case they are invalid, but it has
to be run only *after* the cache is loaded.

Refactor `CacheDistributor` a bit such that the validation (and possible
recreation) of virtualenvs happens only after the cache is loaded.