internal/postgres: fix importGraph to insert fewer modules
internal/postgres: fix importGraph to insert fewer modules

The importGraph function was supposed to insert two modules,
but it was inserting many.

This change speeds up TestSearch from about 40 seconds to
about 5 seconds.

Change-Id: Ie5a3d8fe20056fe0b63ad8cc8b5f86cc6811b156
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/290670
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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