Move from MutableArtifacts.toml to Scratch.jl?

[KristofferC]

In general, package directories are supposed to be immutable and static. This makes it for example work with PackageCompiler. However, trying to run the tests on a package compiled instance of Gtk (https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/dd0c14b_vs_dd0c14b/Gtk.primary.log) gives

fatal: error thrown and no exception handler available.
InitError(mod=:Gtk, error=Base.SystemError(prefix="opening file "/home/pkgeval/.julia/packages/Gtk/vBOOU/MutableArtifacts.toml"", errnum=2, extrainfo=nothing))
#systemerror#79 at ./error.jl:176
systemerror##kw at ./error.jl:176
systemerror##kw at ./error.jl:176
#systemerror#78 at ./error.jl:175 [inlined]
systemerror at ./error.jl:175 [inlined]
#open#658 at ./iostream.jl:293
open##kw at ./iostream.jl:282 [inlined]
#open#659 at ./iostream.jl:355
open at ./iostream.jl:355
jfptr_open_41474 at /sysimage/sysimg.so (unknown line)
_jl_invoke at /workspace/srcdir/src/gf.c:2304 [inlined]
ijl_apply_generic at /workspace/srcdir/src/gf.c:2486
#open#369 at ./io.jl:382
open at ./io.jl:382 [inlined]
#bind_artifact!#5 at /workspace/srcdir/usr/share/julia/stdlib/v1.8/Pkg/src/Artifacts.jl:245
bind_artifact!##kw at /workspace/srcdir/usr/share/julia/stdlib/v1.8/Pkg/src/Artifacts.jl:188 [inlined]
__init__ at /home/pkgeval/.julia/packages/Gtk/vBOOU/src/Gtk.jl:117
...

The issue is https://github.com/JuliaGraphics/Gtk.jl/blob/530da45c952cdc88671a8e739d09cd94e3b5639c/src/Gtk.jl#L91 where the absolute path of the package is stored during precompile time of the package.

I think it would be preferable to move to use Scratch.jl (https://github.com/JuliaPackaging/Scratch.jl) where there is no assumption about the path of the package itself.

cc @staticfloat

[staticfloat]

I've started the process here: https://github.com/JuliaGraphics/Gtk.jl/pull/619

I've also simplified some other things; once we can verify that this simplified process works, let's take a second look at whether this fixes relocation issues properly. There are definitely some issues with that (gdk-pixbuf-query-loaders generates absolute paths) but if the scratchspace is just not copied from machine to machine and gets generated on first run, that should be sufficient.