Support relative paths for registries

[fluffysquirrels]

Closes #2252

I haven't finished the tests for this yet as I wasn't sure what was necessary. Is it OK to just test through registry_configuration() in src/cargo/ops/registry.rs or are integration tests required? If integration tests then I'm not sure what cargo sub-command I can run without a real remote registry to play with; any thoughts?

[rust-highfive]

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[fluffysquirrels]

I wrote a few integration tests for this that pull dependencies from a registry with a relative path.

[ehuss]

I'm a little concerned about changing the definition of registry.index to a URL or file path. Can we keep it strictly URLs only? I think file: URLs should work with relative paths (for example file:alternative-registry, which seems to work as-is in this PR). Perhaps just do Url::parse() on the value just to verify that it is a url before passing it to Url::options().

[fluffysquirrels]

@ehuss ,

OK, makes sense to me, I'll push an update.

Today I learned about relative URLs with a file: protocol prefix!

[fluffysquirrels]

@ehuss, I've pushed a new version with your suggestions.

[ehuss]

Thanks!
@bors r+

[bors]

📌 Commit 987f19d has been approved by ehuss

[bors]

⌛ Testing commit 987f19d with merge 6f39916...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: ehuss
Pushing 6f39916 to master...