Unlocked deps

[macovedj]

This PR servers as an initial description of what the desired behavior of an unlocked-dep would include. As exemplified in the PR, most language toolchains would probably use it in combination with nested interfaces, which is currently in the process of being implemented.

The primary motivation behind this syntax is enabling a way for registry tooling concerned with package resolution to resolve implementations of components (rather than wit). This should hopefully enable workflows where users can depend on specific components, rather than interfaces whose implementations need to be specified at a later time.