[ty] Rejigger workspace symbols for more efficient caching
In effect, we make the Salsa query aspect keyed only on whether we want
global symbols. We move everything else (hierarchical and querying) to
an aggregate step *after* the query.
This was a somewhat involved change since we want to return a flattened
list from visiting the source while also preserving enough information
to reform the symbols into a hierarchical structure that the LSP
expects. But I think overall the API has gotten simpler and we encode
more invariants into the type system. (For example, previously you got a
runtime assertion if you tried to provide a query string while enabling
hierarchical mode. But now that's prevented by construction.)
