The scripts are a lot smaller now that we build fewer images and don't
use jhbuild, so it's small enough to keep in one file. That makes it
easier to read.

We use "mozjs" everywhere else, so to remove any confusion about the
"SM" and "spider" abbreviations, rename the Docker images and the mozjs
build directory.

To the best of my knowledge, no remaining jobs used jhbuild for
anything. We can remove the "DEV: jhbuild" case from everywhere and also
remove jhbuild from the Docker image builds. (The Docker images do still
install SpiderMonkey into /root/jhbuild/install, but we will change that
in the next phase.)

Since we no longer build m4-common with jhbuild in the Docker image
build, we need to install the autoconf-archive package instead.

I forgot what it was for, and had trouble figuring it out, so it should
probably be renamed to something more clear.

Since SM60 is ready to merge, we need to switch to it.

[skip ci] it doesn't change GJS itself.

[skip ci]

The build process requires its own project. So, remove all unneeded
files.

Break it into a set of modules.