x/pkgsite: UpsertSearchDocuments should not store synopsis for non-redistributable packages

[jba]

If a package is not redistributable, we can't store its doc synopsis. That decision should happen at the DataSource (that is, DB) level, in UpsertSearchDocuments.

It's not quite a trivial change because we also have to consider the flag for bypassing license checks, which UpsertSearchDocuments currently can't see because it's on the postgres.DB struct.

[gopherbot]

Change https://golang.org/cl/250637 mentions this issue: internal/postgres: enforce redistributability and allow bypass for search