Fix dependency cache inconsistency with prerelease versions

[chriskuehl]

This PR fixes an inconsistency identified in #7950 (comment) where in some situations Poetry will not select a prerelease package version even though it satisfies all constraints.

Poetry is supposed to consider prerelease package versions if no other package versions satisfy the constraints. I believe this works if the constrained dependency is at the top level, but not necessarily if the constraint is introduced at a later level. Currently, whether it works or not depends on some arbitrary factors like the order dependencies are evaluated in and if the solver recently backtracked.

Regression test

I introduced a regression test which fails on master:

____ test_solver_with_dependency_and_prerelease_sub_dependencies_increasing_constraints _____
Traceback (most recent call last):
  File "/home/ckuehl/proj/poetry/src/poetry/puzzle/solver.py", line 155, in _solve
    result = resolve_version(self._package, self._provider)
  File "/home/ckuehl/proj/poetry/src/poetry/mixology/__init__.py", line 18, in resolve_version
    return solver.solve()
  File "/home/ckuehl/proj/poetry/src/poetry/mixology/version_solver.py", line 111, in solve
    self._propagate(next)
  File "/home/ckuehl/proj/poetry/src/poetry/mixology/version_solver.py", line 150, in _propagate
    root_cause = self._resolve_conflict(incompatibility)
  File "/home/ckuehl/proj/poetry/src/poetry/mixology/version_solver.py", line 351, in _resolve_conflict
    raise SolveFailure(incompatibility)
poetry.mixology.failure.SolveFailure: Because no versions of a match !=1.0
 and a (1.0) depends on B (>0.9.0), every version of a requires B (>0.9.0).
So, because no versions of b match >0.9.0
 and root depends on A (*), version solving failed.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ckuehl/proj/poetry/tests/puzzle/test_solver.py", line 1222, in test_solver_with_dependency_and_prerelease_sub_dependencies_increasing_constraints
    transaction = solver.solve()
  File "/home/ckuehl/proj/poetry/src/poetry/puzzle/solver.py", line 72, in solve
    packages, depths = self._solve()
  File "/home/ckuehl/proj/poetry/src/poetry/puzzle/solver.py", line 161, in _solve
    raise SolverProblemError(e)
poetry.puzzle.exceptions.SolverProblemError: Because no versions of a match !=1.0
 and a (1.0) depends on B (>0.9.0), every version of a requires B (>0.9.0).
So, because no versions of b match >0.9.0
 and root depends on A (*), version solving failed.

The test passes if I disable the DependencyCache (replace the contents of DependencyCache._search_for() with return self.provider.search_for(dependency), and also passes with the patch provided in this PR.

Additionally, the reproduction using black<22 from #7950 (comment) also succeeds for me now.

[radoering]

LGTM. 👍 Just something about our (implicit) internal coding style considering tests.

[github-actions]

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