[vcpkg-get-python-packages] Fix exported variable

[dg0yt]

Noticed while trying to use another variable name then PYTHON3 with lensfun.
In addition:

• fix typos
• make precompiled archive optional.

[dg0yt]

vcpkg/ports/vcpkg-get-python-packages/x_vcpkg_get_python_packages.cmake

Lines 78 to 80 in 11e79b1

	if(arg_PYTHON_VERSION STREQUAL 3)
	file(COPY "${python_dir}/python3${PYTHON_VERSION_MINOR}.zip" DESTINATION "${venv_path}/Scripts")
	endif()

This file isn't installed by python3:x64-windows. So the function still breaks when used with the executable from that port.
https://dev.azure.com/vcpkg/public/_build/results?buildId=98885&view=logs&j=7922e5c4-0103-5f8f-ad17-45ce9bb98e80&t=66eef212-08d6-5e6c-a560-7d01149d0188
CC @Neumann-A

[Neumann-A]

file(COPY "${python_dir}/python3${PYTHON_VERSION_MINOR}.zip" DESTINATION "${venv_path}/Scripts")

probably needs to be generated by the python port itself. Ran already into several issues with embedded python not finding the default scripts but need a good way to compose other modules into that.

[dg0yt]

Hm. I wonder if the actual problem with this port/function is that pip install bypasses asset caching.

[Neumann-A]

It should only install into the venv not into the installed tree.

[dg0yt]

It should only install into the venv not into the installed tree.

Yes. And still it is a problem for disconnected builds after --only-downloads. Unlike most tools which also aren't installed into the installed tree.

Pip has its own caching. And it is also possible to pre-download to a local downloads dir and to install later from that dir:
https://pip.pypa.io/en/stable/user_guide/#installing-from-local-packages
Maybe this will eventually need an injection point and instruction on how to prepare and pass in the local dir. Which may require to build python3 first to get matching downloads.

(I'm also looking at jinja2: Repeatedly popping up now as a libsystemd requirement since this port was allowed to enter this registry, https://github.com/microsoft/vcpkg/issues?q=is%3Aissue+libsystemd+jinja2)