Use correct Poetry config when collecting Poetry projects (#447)

* Use correct Poetry config when collecting Poetry projects

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.

* Fix typo: saveSatetSpy -> saveStateSpy

* poetry: Support same virtualenv appearing in multiple projects

* Add nested Poetry projects test

* poetry: Set up environment for each project individually

* tests/cache-restore: Do not look for dependency files outside `data`

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.

* poetry: Simplify `virtualenvs.in-project` boolean check

* README: Explain that poetry might create multiple caches

* poetry: Run `poetry env use` only after cache is loaded

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.

* poetry: Bump cache primary key

README.md

@@ -55,7 +55,7 @@ The action defaults to searching for a dependency file (`requirements.txt` for p
 
  - For `pip`, the action will cache the global cache directory
  - For `pipenv`, the action will cache virtualenv directory
- - For `poetry`, the action will cache virtualenv directory
+ - For `poetry`, the action will cache virtualenv directories -- one for each poetry project found
 
 **Caching pip dependencies:**