internal/postgres: change symbolsearch results sort
internal/postgres: change symbolsearch results sort

The symbolsearch results are now sorted by NumImportedBy decreasing,
then package path, then symbol name, for consistency with the sort logic
in the queries.

For golang/go#44142

Change-Id: I97b3e0ce1cd720d4775e6e1795506587b21ecb77
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/339610
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite