Add dependency root support

[rtimush]

Currently, protofetch required protofetch.toml to be in the root of the repository.
This is not particularly friendly to the cases when proto files are only a part of what's in the repository, and does not work at all if we want to have more than one protofetch module in one repository.

This PR adds a new optional root field to the dependency definition in the module descriptor. When set, it is treated as if it was the root of the repository — the descriptor is loaded from this path, content roots and policy settings apply relatively to the root.

While root looks similar to content_roots, I don't think we could get away with something like just loading additional descriptors from all specified content_roots. Unlike the new setting, content_roots does not apply any filter to the files, it only makes protofetch remove the prefix from matching files, but proto files outside of the content roots are still included.

I also considered making root a part of the URL, but there is no common way to do so. The most natural approach would probably be github.com/org/repo/path but this makes URL invalid when open in a browser, which could be confusing.

The change is not forward compatible: old protofetch versions will ignore the root field, fetching wrong protos (or most likely rather not fetching the right ones). I'm not sure if we have a good way to deal with such incompatibilities though. Maybe we can bump a version in the lockfile — it won't fix all issues, but at least we will make sure that protofetch fetch --locked always produces the same results.

[smosca-coralogix]

I also considered making root a part of the URL, but there is no common way to do so. The most natural approach would probably be github.com/org/repo/path but this makes URL invalid when open in a browser

They are a nuisance to open, but these kinds of URLs are already very common in Go. I must say that I would like this solution a bit more because of this precedent, but definitely not enough to say it is always better