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 the rust-analyzer.toml files that can be
placed under 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.
a15cc86c64