External debug symbols for snapshot symbol selection

Sometimes people package up debug symbols separately from the executable. I'm not sure how that works yet, but we should definitely support it.

Start by tracing how snapshot symbol selection currently finds symbols and investigate how separately packaged debug symbols are identified and associated with an executable. The issue names no files or tests; done means snapshots can resolve symbols from external debug-symbol packages, with coverage for the supported packaging cases.

Written by the indexing model from the issue text.