libxml2: don't propagate Python bindings

[alyssais]

Motivation for this change

Extracted from #77149. Quoting myself there:

I suspect there will be packages that relied on the libxml2 Python
bindings being propagated, but don't know how I'd identify them. The
libxml2 change is too big for nix-review to handle. Any ideas?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[FRidh]

This has existed since about 2015, 38313d5, which was when multiple outputs was introduced (#7701).

Nowadays the Python bindings are in python-packages.nix and packages should use those. Not all of them work that way though.

[jtojnar]

If the docs are correct, we should not need this line at all:

propagatedBuildOutputs of that package which by default contain $outputBin and $outputLib

[jtojnar]

Looks like that is still he case:

nixpkgs/pkgs/build-support/setup-hooks/multiple-outputs.sh

Line 182 in a8d1f32

local po_dirty="$outputBin $outputInclude $outputLib"

[jonringer]

i removed the whole line on master and was able to build a few hundred packages fine :)

[alyssais]

Okay then. Here we go!

I guess the staging process would pick up if this did cause catastrophic breakage anyway.