Cleanup vendored dependencies

[abn]

This pull request contains 3 relevant changes to package vendoring.

1. Remove unused vendored dependencies.
2. Debundle compatibility only dependencies. These are only required in smaller scenarios and will be dropped by poetry 1.3 releases anyway.
3. Prototype sys.path injection I am not sure how reliable this is, feedback would be nice. Considering poetry use case in isolated, I do not expect this to have any issues. This would also mean that distribution package maintainers can debundle extremely easily by dropping the _vendor direcotry and removing the injection. We could go further by testing if the directory exists before pre-pending.

Relates-to: #2346

[sdispater]

While I agree it would be better in theory, I wonder if this won't conflict with what we do in Poetry, and if the vendors of poetry-core will not take precedence.

[abn]

Good question, I want to do a bit more testing on this one actually. We could move it into poetry/__init__.py. Although, I am not a 100% on the ordering.

One thing I was wondering if we could provide a kind of proxy instead that can be smart enough to pick system one if the vendor site is not available. That would mean it would achieve what we want, while maintaining explicit imports.

[abn]

For posterity, if the sys path option does need to be replaced later. This can be an alternative.

import sys
import importlib

from poetry.core.utils._compat import Path

__all__ = []
__vendored__ = {}

try:
    # poor developer's requirements parser
    with open(Path(__file__).parent / "vendor.txt") as f:
        for line in f.readlines():
            name, version = line.split("==")
        __vendored__[name] = version
        module_name = "poetry.core._vendor.{}".format(name)

        try:
            locals()[name] = importlib.import_module(module_name)
        except ImportError:
            locals()[name] = importlib.import_module(name)

        sys.modules[module_name] = locals()[name]
        __all__.append(name)
except (FileNotFoundError, ValueError, IndexError) as e:
    pass

[pradyunsg]

At that point, it might make sense to just switch to using pip's internal helper: pip._vendor:vendored.

[sdispater]

Looks good to me 👍