Use Salsa infrastructure for find-references and rename

[lionel-]

Branched from #1254
Progress towards #1212
Closes #1149

This PR moves find-references and rename onto the Salsa infra and deletes the old implementation (the textual workspace walk).

Candidate search: Unlike Rust-Analyzer that does a textual search across the workspace and resolves all occurrences, we're more aligned with ty: we filter files to inspect with a textual search, but then use a post-parse structure to filter occurrences. ty walks the AST, we walk the index instead with a new uses_of() method that is more to the point than the raw AST.

Candidates are then narrowed with File::resolve_at() to check that they match the definition of the symbol at point.

New supporting infra:

• Salsa-based Identifier::classify()
• all_files() (Salsa query) that returns all files known to the database
• root_by_file() that disambiguates to which workspace root a file belong when roots are nested. That is meant to be the one source of truth for root ownership of files.

Removed infra: The Document copies of the Rowan parse tree and semantic index are now removed, in favour of the Salsa DB.

Other fix: Goto-definition now benefits from the new Salsa-based Identifier::classify() which snaps the cursor to the symbol at point. Fixes goto-def when cursor in on RHS boundary.

I recommend testing with #1259 to benefit from bug fixes and improvements in ulterior branches.