file domain - confusing errors/behavior when using composition

[mildwonkey]

We ran into some confusing behavior - explained by the lack of compose support for relative file paths - when working through Andy's OSCAL parity PRs. I see opportunities for better error messages (and documentation); I don't think there's a bug here (per se) but we do need to clean up the behavior:

The first issue we saw was user error, and a documentation issue: we had file:/some/path instead of file://some/path in the link to the validation, and that syntax isn't referenced in the file domain page at all.

This is the actual thing that isn't working currently (this is known and documented) - that final relative path is the problem:
component-def.yaml -> links to validation (abs path) -> validation links to a file (relative path)

these aren't instructions, but some thoughts on outcomes from this issue:
-[ ] filepath syntax in file domain doc (relative paths, absolute paths, file://, other supported getters)
-[ ] consider checking for file paths that start with something like file: but not file:// (and only if there is an error return a "this looked weird" message)?
-[ ] can we return a clearer error about the file domain when composition is required and it finds relative file paths? Is there a way lula can abort a given validation if it sees a relative file path? (until we implement compose)
-[ ] or just hurry up and fix compose + file domain, @mildwonkey, geez.