Converge JLL APIs a bit more

[staticfloat]

The initial draft of Fake JLLs missed some important points in JLL
API compatibility; let's close the gap somewhat, where we can. This PR
adds the following exports:

• get_artifact_dir(): returns the Julia prefix
• dev_jll(): Throws an error
• best_wrapper: always set to nothing
• get_*_path(): returns the path of the identified library product

It also converts all fake JLL modules to baremodules, and sets the
appropriate compiler options to minimize compilation.

Closes #38781

[staticfloat]

@fingolfin Please let me know if any of these APIs don't work or are insufficient in some way.

[fingolfin]

This also only addresses half of issue #38781 from my POV: there is still the issue of the missing headers...

[staticfloat]

This also only addresses half of issue #38781 from my POV: there is still the issue of the missing headers...

We can't ship the GMP headers in include (since that would conflict with other GMP installs when unpacked to e.g. /usr/local). We could ship them in include/julia, but I'm personally against that. I think the best solution is for whatever build process you're using, to install the full GMP artifact manually on v1.6.

You can do that via something like:

using GMP_jll, Pkg.Artifacts
artifacts_toml = joinpath(dirname(dirname(Base.pathof(GMP_jll))), "StdlibArtifacts.toml")

# If this file exists, it's a stdlib JLL and we must download the artifact ourselves
if isfile(artifacts_toml)
    meta = artifact_meta("GMP", artifacts_toml)
    hash = Base.SHA1(meta["git-tree-sha1"])
    if !artifact_exists(hash)
        dl_info = first(meta["download"])
        download_artifact(hash, dl_info["url"], dl_info["sha256"])
    end
    global gmp_artifact_dir = artifact_path(hash)
else
	# Otherwise, we can just use the artifact directory given to us by GMP_jll
	global gmp_artifact_dir = GMP_jll.find_artifact_dir()
end

[StefanKarpinski]

Good to merge, @staticfloat?