packaging: allow installing without git

[branchvincent]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

Related to #4145

This allows building/installing snapcraft from non-git clones. It would be useful to Homebrew, as we prefer to build from lightweight tarballs.
If git describe fails, it first checks if the version is defined in the environment via SNAP_VERSION or falls back to a PEP440-compliant version string 0.0.0+devel (this is similar to the fallback in snapcraft/__init__.py, but updated for PEP440).

Before:

$ pip3 install https://github.com/snapcore/snapcraft/archive/refs/tags/7.5.0.tar.gz
...
      subprocess.CalledProcessError: Command '['git', 'describe', '--always', '--long']' returned non-zero exit status 128.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error
...

After:

$ pip3 install https://github.com/branchvincent/snapcraft/archive/refs/heads/build-no-git.tar.gz
...
Successfully installed snapcraft-0.0.0+devel
$ pip3 uninstall -y snapcraft
$ SNAP_VERSION=7.5.0 pip3 install https://github.com/branchvincent/snapcraft/archive/refs/heads/build-no-git.tar.gz
...
Successfully installed snapcraft-7.5.0

[sergiusens]

thank you for this!

[codecov-commenter]

Codecov Report

Merging #4306 (c9ae8d8) into main (91a18b3) will not change coverage.
The diff coverage is n/a.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@           Coverage Diff           @@
##             main    #4306   +/-   ##
=======================================
  Coverage   89.08%   89.08%           
=======================================
  Files         300      300           
  Lines       20571    20571           
=======================================
  Hits        18325    18325           
  Misses       2246     2246           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more