internal/postgres: refactor runSymbolSearch
internal/postgres: refactor runSymbolSearch

runSymbolSearch is refactored such that the majority of its logic is
split into fetchMatchingSymbolIDs and fetchSymbolSearchResults.

This is done in preparation for changing the multiword symbol search
logic.

For golang/go#44142

Change-Id: I9a59edad79dd846da11f7f73777805f1de7af599
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/338449
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