pkg_resources has long been deprecated, but until recently has

remained consistent in behavior. The replacements can be found in

importlib.resources, importlib.metadata, and packaging modules.
Over time, pkg_resources began to wrap newer versions of some of these

modules, breaking backwards-compatibility. While not ideal, it's

incentive to move toward the modern packaging support in Python.
This change updates djblets.extensions to (almost) fully move to these

new packages. In some cases, the updates are fairly straight-forward,

but in others there's some rewriting that needs to take place:


Version parsing is now strict, so we need to better bullet-proof

  against parsing issues.


Metadata is now handled by importlib.metadata, allowing us to remove

  a bunch of old code.


The old extension package installation code had to be wired off. This

  was never truly supported, couldn't install wheels, and had a big

  scary warning in the docs, so we're not going through a deprecation

  cycle for this.


The packaging support still uses pkg_resources, as that's what

setuptools.Command uses. This will need to be converted in time.
There may be some fallout from this change, as there may be behavioral

differences in results/error handling in some cases. If any issues come

up from this change, we'll need to identify the differences and

proactively address them.

Djblets and Review Board unit tests pass. Tested on Python 3.7 and 3.12

with the latest versions of packaging, importlib_metadata, and

importlib_resources packages.
Tested Review Board with loading and using extensions (integrations,

Review Bot, and Power Pack).