Cargo plugin can't handle git sources

[gtristan]

This is a clone of the same issue reported against the cargo plugin for bst-1: https://gitlab.com/BuildStream/bst-external/-/issues/48

Some Cargo.lock files can specify dependencies through git repositories, but the cargo plugin is unable to handle this case.

[sophie-h]

I looked into this and it seems to be a bit tricky. The problem is that kind: cargo can only have cargo plugin refs and not git refs.

The cargo plugin has to introduce a ref format for git sources. That's not really complicated. That, however, means that for all functionality like download etc., the cargo plugin also has to implement the git functionality. It might be possible to do this by wrapping around the git plugin in these cases. But I'm not sure if that's the right way to do this.

[sophie-h]

I'm working on this now.

[sophie-h]

This feature is now available in buildstream-plugins-community as part of the cargo2 plugin.