add a Source::parse arm for sparse repos (keeping it in the scheme). …

[sthornington]

Proc macro resolution wasn't working properly for sparse repos. This fixes #2938.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[sthornington]

I wrote a test for it, but was unable to run it - cargo test seemed to want some runfiles stuff and looking at that in crates.io lead me to believe that these things are not able to be tested from within cargo.

[illicitonion]

LGTM, thanks so much!

[sthornington]

Thanks, let me know if there is anything else for me to do to get this merged!