configure --enable-system-site-packages: First check all non-site packages

[mkoeppe]

We first test all the packages without SAGE_PYTHON_PACKAGE_CHECK, then those with SAGE_PYTHON_PACKAGE_CHECK.

This is purely cosmetic, but possibly also useful preparation for checking for the site packages faster, using only one venv, and with less repeated output.

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[github-actions]

Documentation preview for this PR (built with commit 9871ad6; changes) is ready! 🎉

[orlitzky]

This part works fine.

I thought about trying to optimize the repeated venv creation, but in the end, wasting time by repeating checks is the autotools way. (It also keeps the logic simpler and would make failed checks easier to debug.) But I was on the fence about the tradeoffs, so if you want to do it the other way, I don't mind.

[mkoeppe]

Thanks!