The importlib.metadata and pkg_resources backends unfortunately
normalize extras differently, and we don't really want to continue using
the latter's logic (being partially lossy while still not compliant to
standards), so we add a new abstraction for the purpose.
The typing module has been available since Python 3.5. Guarding the
import has been unnecessary since dropping Python 2.
Some guards remain to either:
- Avoid circular imports
- Importing objects that are also guarded by typing.TYPE_CHECKING
- Avoid mypy_extensions dependency
The stdlib module has been available since Python 3.5 and the
TYPE_CHECKING constant has been available since 3.5.2.
By using stdlib, this removes the need for pip to maintain its own
Python 2 typing compatibility shim.
pkg_resources performs annoying caching that needs to be worked around
in some parts of pip. This makes it easier to represent the difference
between environments backend by WorkingSet() and working_set.