Revise how we handle sysinfo.gap and gac / replace Setup.regenerate_gaproot

[fingolfin]

Based on a comment on PR #1079, I think it is time to revise or indeed replace Setup.regenerate_gaproot. It is now reduced to generating an additional GAP root dir containing a sysinfo.gap and a gac executable.

It wastes some time each time GAP is started to find a C and C++ compiler, even if none is needed.

Perhaps time now to transform this:

• produce a function which takes a directory path and puts sysinfo.gap and gac (and whatever else) into it
• then only run this function on a fresh temp dir just before invoking GAP.Packages.install (perhaps caching the resulting dir for the duration of the GAP.jl session)
• perhaps create_gap_sh should also make use of it, and produce a gac and sysinfo.gap which fit together with the produce gap shell script (towards resolving Invalid path to GAP executable in sysinfo #1105)
• actually with that last bit we could circle back to the top: before calling GAP.Packages.install perhaps we should call this enhance create_gap_sh and make sure PackageManger "sees" its sysinfo.gap (and of course gac, gap)

This would also hopefully allow us to deal with the three packages listed under "Missing Software: GAP" in #1122

This would probably also resolve #840