Install stg executable using entrypoint mechanism

Install stg executable using entrypoint mechanism

When a stgit distribution is installed, the stg executable will now be
auto-generated at install time by the installer (i.e. pip). This is a
standard, contemporary approach for python applications. It helps ensure
that the executable is associated with the correct python runtime
environment.

The `stg` script at the worktree root remains, but only as a
development-time convenience.

To re-enable code coverage, the t/test-bin/stg script is added and
test-lib.sh updated. This ends up being a nice simplification versus the
previous approach with the really nice benefit of no longer having to
comprehend coverage within the stgit python code. t/test-bin/stg is now
able to wrap stg invocations with coverage.

N.B. test-lib.sh used to complain mightily when it detected that stgit
wasn't previously built. Those complaints were removed. The test suite will
run just fine if the build hasn't happened, with the caveat that cmdlist.py
will not have been generated and thus there is more runtime overhead. So
its still best to build to improve test runtime.

Signed-off-by: Peter Grayson <pete@jpgrayson.net>