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.

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[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!

[sthornington]

Thanks everyone for getting this in. Can I ask for a point release which includes this, or when is 0.53.1 otherwise planned? Or can I do start it myself? Thanks again.