Add relative paths to metadata for post-install script execution

[ncoghlan]

Adding the local-export support showed that using the venv Python to run postinstall.py to regenerate pyvenv.cfg is unreliable on Windows (as whether or not it works depends on exactly how Python is set up in the Windows installation). It only works on other platforms because the Python "binary" in the venv is a symlink to the real binary in the base runtime environment.

The local-export feature works around that by using the already set up build environment to run the exported environment's postinstall.py.

test_minimal_project relies on the fact there's exactly one runtime environment defined in that test case, and falls back to using that if using the unpacked environment's own Python binary fails.

The archive and environment metadata should be updated to include three new fields, all relative to the installed environment's base folder rather than to the metadata file:

• postinstall_script: path to post-installation script (usually just postinstall.py, but allows migration to a different name without breaking compatibility). Set for all layers.
• base_python: path to the base runtime Python for that environment (for layered envs, ../<base_env_name>/python.exe on Windows, ../<base_env_name>/bin/python elsewhere; for base runtimes, python.exe on Windows, bin/python elsewhere). Set for all layers.
• app_python: path to Python inside the env (Scripts/python.exe on Windows, bin/python elsewhere). Only set for application layers.

With those fields added, local environment exports and consuming applications can always use base_python and postinstall_script to finish the export process, while app_python can be used to run application launch modules.

[ncoghlan]

For ease of access, it may be worth saving a subset of the layer metadata to the share/venv/metadata folder proposed in #17.

Specifically, starting from unpacking the application layer archive, an installer should be able to work out:

• which framework environments need to be available
• which base runtime environment needs to be available
• how to run the post install script
• how to run the application launch module

Each framework environment would need to report:

• which base runtime environment needs to be available
• how to run the post install script

And each base runtime would need to report:

• how to run the post install script

Suggested file name would be share/venv/metadata/venvstacks_layer.json

[ncoghlan]

Another enhancement this change would unblock is to make even layered environment archives fully compatible with the tar_filter extraction filter.

While #23 adds that for the base runtime archives, layered environments currently still rely on external symlinks to the underlying Python implementation.

If postinstall.py were to always be executed with the base runtime directly, then the external symlink could be omitted from the archve, and the app_python and base_python metadata fields could be used to recreate the missing symlink from the post-installation script.

Framework layers wouldn't need to recreate the symlink, since everything executed on a deployed system should be running in an application layer environment.

[ncoghlan]

The share/venv/metadata/venvstacks_layer.json part of this is being implemented to resolve #66