coping with with as-yet-untyped poetry.core, and different python
versions

This change ensures that poetry does not bail out when encountering a
bad distributions in `sys_path`. This behaviour is similar to how `pip`
handles this today. In addition to ignoring these distributions, we
also issue a warning log so users can choose to act on this.

Further, this change also handles a scenario where an empty path is
present in the `sys_path`.

Since commit f5cbba2b (#4766),
test_executor_should_write_pep610_url_references_for_git has been
writing a different version of this file than the one checked into the
repository, leaving a dirty working tree.  Fix it and add a CI test.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Decorating class instances methods using `lru_cache` prevents garbage collection
to happen when an object gets deleted. This could be really bad for memory
consumption.
Some resources about this:
- https://rednafi.github.io/reflections/dont-wrap-instance-methods-with-functoolslru_cache-decorator-in-python.html
- https://www.youtube.com/watch?v=sVjtp6tGo0g

Originally spotted by rule `B019` of `flake8-bugbear`.

solver: print more information about packages with source_type when solving fails to avoid weird messages like "Because myapp depends on both demo (1.2.3) and demo (1.2.3), version solving failed."

This change is required to allow for subdirectories.

Co-authored-by: Bartosz Sokorski <b.sokorski@gmail.com>
Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>

Fixes #2355

This works around an issue with cleo's `Command.choice` implementation.
If the user does not make a selection then an AttributeError is raised.

This change ensures that the provider is less eager when merging
duplicate dependencies for a package. In particular, takes into
consideration source type, url and reference if available.

Use the version from poetry.core always, one implementation is enough

Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>