build: cleanup base files inclusion

[vtjnash]

Ensures the out-of-tree build has the correct src files installed for the base image, and simplifies some of the includes used in Base.

Fixes #31140

[vtjnash]

I will assume people are okay with this, and take review comments afterwards if not

[Keno]

I don't really understand what the point of moving the files from usr/share/julia/base to usr/share/julia/src/base is? That seems like a completely random directory structure.

[vtjnash]

It sort of is random, in that the alternate versions I tried seemed to have more problems with make or Revise getting confused. I didn't really want to nest them this way for any design reason.

[Keno]

Revise is very confused right now :).

[timholy]

Revise is fixable (see timholy/Revise.jl#694), but it would be worth knowing whether this change is going to be reconsidered. I wouldn't choose an unfortunate directory structure in a (failed) attempt to keep Revise happy, I'd rather see this solved as it should be and Revise can adapt accordingly.

Revise always breaks with each point release, and this is comparatively easy. It's the changes that affect JuliaInterpreter that are scary.

[timholy]

Quick thoughts from @vtjnash? Should we change this directory organization or should I merge timholy/Revise.jl#694?

[staticfloat]

I would very much like to change the directory structure. This change confuses not just Revise but also VSCode, grep, and other tools.

[pfitzseb]

The directory structure is probably fine, but symlinking usr/share/julia/src/base to src/base makes sense imo.

[timholy]

I'll hold off on the Revise change then. I'll also defer to others in implementing the change since everyone involved in this conversation has a better sense for what needs to be done than me. We'll update Revise when it's done. (@pfitzseb feel free to just modify your PR and merge it whenever you wish.)