Replace relative imports by absolute ones in sage.libs except .ntl

[mkoeppe]

• Cherry-picked from Modularization of sagelib: Distributions sagemath-{brial,combinat,eclib,flint,gap,giac,glpk,graphs,groups,homfly,lcalc,libbraiding,libecm,linbox,modules,mpmath,ntl,pari,plot,polyhedra,schemes,singular,standard-no-symbolics,symbolics} #35095
• This is for Use Cython 3 implicit namespace package support #36228

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[tobiasdiez]

What is this file?

[mkoeppe]

slipped in while cherry picking, I'll fix it

[github-actions]

Documentation preview for this PR (built with commit 486508d; changes) is ready! 🎉

[tobiasdiez]

LGTM. For the next time, please just add another commit fixing the issue instead force-pushing everything. As a referee I don't want to check everything again, only the changed parts (i.e here that you removed the file). Thanks

[mkoeppe]

As these commits are cherry-picks from #35095, it's more important for me to keep the commits in clear relation to the original commits.