Provide sensible defaults when using --source-service argument

[franklinsch]

docc added support for specifying information about the remote source repository where the project is hosted (e.g., its GitHub URL) in swiftlang/swift-docc#256, so that it can automatically emit a link to the symbol's source code.

Example invocation:

swift package generate-documentation \
  --source-service github \
  --source-service-base-url https://github.com/<org>/<repo>/blob/main \
  --checkout-path <path to local checkout>

Since SwiftPM already has smarts about the repo in which the package resides, the plugin should be able to fill in some of the arguments above by default. For example, it could pass the root path of the current repository to the --checkout-path argument. Maybe it could also find the appropriate --source-service-base-url value by inspecting the git remote's URL. I'm not sure whether these values can be grabbed using the current plugin API—it's possible there it additional information that SwiftPM would need to surface to make this work.

Importance:

This feature would make it easier to automatically include links back to documented symbols' source code on remote source services like GitHub. It's not critical though, because the values you need pass to DocC are pretty much constant and can be provided via a wrapper script.