Mirrors

[fosslinux]

Here's a first attempt at a mirrors network as per #485.

This is quite basic, and intended to be iterated upon to possibly add things like IPFS/torrents, mirror discovery, etc.

At this point in time, this PR only attempts to implement the following model:

• mirrors are given by a static list as a configuration variable
• each mirror has a copy of all distfiles, and creates git archive snapshots

The "sources" format has a change: there is now a Git line supported, which has the format:

git://<Git url> <url to a Git archive snapshot, if one exists, or else an underscore> <checksum of the Git archive snapshot> [filename]

download-distfiles.sh, rootfs.py and helpers.sh have been updated to reflect this.

mirror.sh is an implementation of a mirror creation/maintenance tool, it is intentionally written in POSIX sh (not bash, like we use for most other things), because platform diversity should be encouraged. I would also encourage other mirror implementations to be written eventually.

Alongside that, in this PR I have accordingly swapped out a number of distfiles to get us "closer to upstream";

• all gnulib snapshots have been changed to git archive snapshots
• all GitHub snapshots (not release artifacts, which could be non-dynamically generated tarballs) have been changed to git archive snapshots; namely, gnu-autogen-bootstrapping in autogen, guile-psyntax-bootstrapping in guile, kexec-tools
• other Savannah snapshots, namely Autogen and coreutils, have been changed to git archive snapshots
• bash-2.05b, bc-1.07.1, bzip2-1.0.8, ed-1.4 have all had their tarballs moved back to upstream, rather than recompressed versions created by various distributions
• gcc-10.4.0 and gcc-13.1.0 have moved to mirrors.kernel.org for consistency

As of yet unresolved questions;

• what do we do about CI now? should it use the main mirror network? or should it use a special CI mirror?