Changed all baseline rustdoc generations to use placeholder manifest

[tonowak]

This PR is a continuation of #340.

This PR changes the way baseline rustdoc of PathBaseline is being generated -- similarly to RegistryBaseline, a placeholder project is created (with a dependency to the true project) on which the rustdoc is generated.

I was worried about two possible places which could have bugs, but after writing this PR, it turned out to not be a problem:

• the dependency is given an absolute path, instead of a relative path, so the target directory in which the placeholder project is placed doesn't matter,
• getting all features from a manifest (instead of the registry) turned out to be extremely similar to getting all features from registry. I've split the feature functions into multiple smaller functions, which shows similarities for easier comparison and lesser chance of making a bug.

[obi1kenobi]

I like the direction. Just needs some polishing for maintainability.

I'd also feel a lot more comfortable with these changes if we had automated tests over them that use path and gitrev arguments in the CLI. Consider adding a new job that looks to catch one of the known (but not yanked) semver violations by path and by gitrev, and ensures the output shows the semver issue was found?

[tonowak]

Consider adding a new job that looks to catch one of the known (but not yanked) semver violations by path and by gitrev, and ensures the output shows the semver issue was found?

Do you have some specific crates / versions in mind?

[obi1kenobi]

Do you have some specific crates / versions in mind?

I know the affected version for clap is still up, it's a solid candidate.

[obi1kenobi]

Looks good so far. The tests are the biggest remaining thing, since I believe our entire test suite currently uses registry-based baselines.

I suggested a couple of tiny incremental improvements as well.

[tonowak]

I know the affected version for clap is still up, it's a solid candidate.

I'm not entirely sure whether I understood what you mean, but I've added a test in the CI. And also I've found a bug in the previous CI :)

[tonowak]

This PR also fixes #344.

[obi1kenobi]

Awesome job!