prepro -- prepare GOPROXY and go-import friendly website

Go modules (https://go.dev/ref/mod), that are not located on hard-coded
well-known hosting platforms like GitHub, are discovered by go-get using
the go-import-protocol: it requests special URL, based on module's name,
to get HTML with <meta name="go-import"> tag. That tag will redirect
go-get further either to VCS, or to GOPROXY-capable URL.

If VCS is in use, then go-get will invoke external utilities to checkout
the code from VCS. After that, it will create ZIP-archive with the
snapshot of the code and store it locally in cache directory.

GOPROXY protocol can be treated just like a HTTP-transport to that cache
directory. Only several prepared static files per each module's version
is enough.

* VCS solution is usually quicker to setup, because development VCS
  repositories are publicly available in most cases too
* VCS requires corresponding external tools to be installed. Hardly
  either Fossil or GNU Bazaar are often installed on many systems
* VCS checkout requires more disk space: for the VCS itself and for the
  cached module's archive
* VCS is slower to check and download in general, because it requires
  several round-trips during the handshake between server and client
* GOPROXY is easier to scale on multiple hosts, because you have to
  replicate only several static files

Another advantage of GOPROXY is actually the reason why "prepro" was
born: current go-get can not handle SHA256-driven Git repositories. It
git-init-s empty repository first and then tries to git-fetch it, that
won't work if git-init was not aware of SHA256-based format. GOPROXY
gives easy ability to make that module's repository publicly fetchable.