pkg_resources is deprecated, and not long for this world. To modernize

our code, we now use importlib (specifically, the importlib_metadata

and importlib_resources compatibility modules) and packaging.
These aren't one-to-one replacements for pkg_resources, but porting

was pretty minimal. For the purposes of RBTools, the new APIs give us

everything we need to look up entrypoints, fetch resource files, and

compare versions.

Tested running built-in RBTools commands, aliases, entrypoint-based

third-party commands, and commands in the search path starting with

rbt-.
Tested generating command line completion scripts.
Tested version parsing for all our flavors of version identifiers

provided throughout Review Board history (old-school 1.0alpha1

through modern 6.0b2).