devtools/cmd/seeddb: filter stdlib versions
devtools/cmd/seeddb: filter stdlib versions

It's relatively expensive to fetch a stdlib module version, so when
std@all is specified, only fetch the minor versions (ones with a patch
version of 0).

Change-Id: Iabf31977ca068a61f7fbbcd38f6da25c14b65082
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/335133
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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