bug#44030: guix import pypi foo@1.2.3 breaks

[Lulu]

I just sent in a patch to fix this for the pypi importer, although
ideally we'd want versioning support for all importers with a uniform
syntax.

I poked and prodded at the RubyGems API to see if it can do what PyPI
can. It's unfortunately much more limited: API v1 provides a method
for querying all versions of a gem [1] and v2 provides a method for
querying specific versions of a gem [2], although neither of them
provides information about dependency versions, so we can't rely on it
for recursive imports (since it would try to import latest versions of
all dependencies). There's a method that fetches dependency and
versioning info for all versions of all given gems but (probably due
to the verbosity of JSON/YAML) it's in binary Ruby serialisation
format [3]. I can try to hack a parser that surgically extracts
dependency info from a given gem version. What do you think?

ELPA runs into a similar problem in that it provides tarballs/files
for older versions but dependency info is only provided in the repo
file. (MELPA tries to directly peg all packages to their respective
Git repos' trunks.)

I presume it's much simpler with the gnu importer, as it's only a
matter of pointing the FTP fetch in the right direction, although I
couldn't confirm it, as the gnu importer doesn't work for me since my
ISP blocks PGP keyserver ports.

I need to take a closer look at CRAN, CPAN, TeXLive and opam.

--
Lulu

[1]: https://guides.rubygems.org/rubygems-org-api/#gem-version-methods
[2]: https://guides.rubygems.org/rubygems-org-api-v2/
[3]: https://guides.rubygems.org/rubygems-org-api/#misc-methods