internal/postgres: sort paths before insertion to avoid deadlock
internal/postgres: sort paths before insertion to avoid deadlock

Deadlock is rare but we have observed it when multiple versions of the
same module are inserted concurrently.

Change-Id: Idd28bd558009ff053ac198bdd310fa780af66f83
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/320750
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