[email protected]: define altinstall? to help changing default python

[branchvincent]

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Currently, swapping the default python (the one aliased to python3) requires carefully copy/pasting logic to remove conflicts from the previous default to the new one. Let's simplify the process by defining a altinstall? helper instead. The only manual changes left can't be dynamic (the link_overwrites and the caveats)

This will help introducing Python 3.14 in #245931