Improve postinstall script resilience

[ncoghlan]

• Allow postinstall scripts to be executed with any Python interpreter (not just the deployed base runtime interpreter)
• Generate layer config file as part of layers
• Use a common postinstall script in all layers
• Generate the deployed sitecustomize.py file from the layer config in the postinstall script
• Add unit tests for the common postinstall script

Closes #66. Implements initial steps towards #19.

[ncoghlan]

Need to avoid dependency on walk_up parameter to relative_to for Python 3.11:

src/venvstacks/stacks.py:1060: error: Unexpected keyword argument "walk_up" for "relative_to" of "PurePath" [call-arg]

[ncoghlan]

Latest Windows failures are due to the fake "absolute" paths in the postinstall tests lacking a drive letter, so they're not considered absolute on Windows.

I also worked out some new checks to add to the check_deployed_environment helper methods in the test suite that would have helped catch these problems earlier:

• Change the get_exported_python callback to report the full environment path (not just its name)
• Require that all reported sys.path entries by located within the parent of the environment path
• The environment path must not be a child of the current working directory
• The environment path must not be a child of the build environment's build path

[ncoghlan]

Even with the new build and deployed environment sys.path checks passing, there appears to still be a problem with the layered environments not correctly picking up the items that are already installed in the lower layers.

[ncoghlan]

There are some relative paths that are not being correctly resolved (they're being resolved relative to the environment path rather than the base build path)