internal: add `library` fixture meta
Currently, there is no way to specify `CrateOrigin` of a file fixture ([this] might be a bug?). This PR adds `library` meta to explicitly specify the fixture to be `CrateOrigin::Library` and also makes sure crates that belong to a library source root are set `CrateOrigin::Library`.
(`library` isn't really the best name. It essentially means that the crate is outside workspace but `non_workspace_member` feels a bit too long. Suggestions for the better name would be appreciated)
Additionally:
- documents the fixture meta syntax as thoroughly as possible
- refactors relevant code
[this]: 4b06d3c595/crates/base-db/src/fixture.rs (L450)
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