internal: Implement parent-child relation for `SourceRoot`s
This commit adds the said relation by keeping a map of type `FxHashMap<SourceRootId,Option<SourceRootId>>` inside the `GlobalState`. Its primary use case is reading `rust-analyzer.toml`(#13529) files that can be placed in every local source root. As a config will be found by traversing this "tree" we need the parent information for every local source root. This commit omits defining this relation for library source roots entirely.
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