Extensible flake references

[edolstra]

Further to #5, it's useful to obtain non-flake dependencies via other schemes than the ones currently built in (i.e. Git, GitHub, and tarballs). For example, we may want to specify a flake dependency such as:

requires = [ pypi:numpy ];

which would mean "get the source of the latest version of numpy from pypi". This would translate into a regular lock file entry such as

        "pypi:numpy": {
            "uri": "https://files.pythonhosted.org/packages/2b/26/07472b0de91851b6656cbc86e2f0d5d3a3128e7580f23295ef58b6862d6c/numpy-1.16.1.zip?hash=sha256-31d3fe5b673e99d33d70cfee2ea8fe8dccd60f265c3ed990873a88647e3dd288"
        }

Some of these should probably be built into Nix (as long as they don't require pulling in crazy dependencies for doing the lookups), but it should also be possible to provide them via plugins.

[CSVdB]

Which options would you want exactly? It would be clearer as an issue to choose a few and make issues for them. We could also wait for the beta and ask people which ones they would be interested in, but this isn't really an "issue" given that it can never really be completed.

[edolstra]

Closing, new input types can be defined by plugins now.