At the heart of the loop for extracting usages across a Sourcegraph instance
is the extractLocationsFromPosition function, which extracts related symbols
and source ranges from a single SCIP Document. (Source ranges for returning
to the user directly, and related symbols to do further lookups, e.g. in the case
of inheritance.)

Since we want to perform matching based on symbol names in the upcoming
precise usagesForSymbol API, and also return symbol names for each associated
source range, this function needs to be updated to:

1. Be able to take a symbol name for doing lookups. This is done using the new
   FindUsagesKey type which allows two cases - position-based and symbol-based.
2. Be able to return symbol names associated with every source range. This is
   done by creating a new UsageBuilder type which somewhat subsumes
   the Location type. We avoid copying the same 'UploadID' and 'Path' fields
   eagerly for clarity; that will be handled by callers in the future when they
   mix UsageBuilder values across different Documents (by first calling build).

For the above, I've introduced a new func extractRelatedUsagesAndSymbolNames,
and extractLocationsFromPosition delegates to that. In the future,
extractLocationsFromPosition will be removed.

Stepping stone for GRAPH-573

Test plan

Covered by existing tests. I will add more tests once we can pass in a FindUsagesKey
with a symbol-based Matcher from some other functions higher up in the call chain.