swift-package-manager

Remove `ExpressibleByStringLiteral` conformance for RelativePath.
This was used to make transition simpler, but can cause confusing
runtime errors (since there are implicit requirements about whether
or not the string starts with a `/` depending on the context).

This change also standardizes the clients to that they use the
`appending(component:)` or `appending(components:)` methods, as
appropriate.