cmd/godoc: send go-source meta tag for golang.org/x repos
cmd/godoc: send go-source meta tag for golang.org/x repos

This allows godoc.org to link to the source code. I chose to link to
github.com instead of googlesource.com because their file browser is
better.

Details here:
https://github.com/golang/gddo/commit/864b1c0aba009e37d301365a2f6f6cf51a3caef3

Change-Id: I962b3fadc822e753245f52240937be5c7e158dc7
Reviewed-on: https://go-review.googlesource.com/2719
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

matryer/tools,magiccao/tools,jdhenke/tools,argandas/tools,cmars/tools,nicot/tools,sshetty10/tools,jdhenke/tools,jdhenke/tools,golang/tools,dmitris/tools,cmars/tools,runcom/tools,dmitris/tools,4ad/tools,4ad/tools,syreclabs/go-tools,runcom/tools,springning/tools,nicot/tools,glasser/golang-tools,argandas/tools,maddyonline/tools,maddyonline/tools,matryer/tools,springning/tools,josharian/tools,springning/tools,cmars/tools,maddyonline/tools,glasser/golang-tools,golang/tools,glasser/golang-tools,cw2018/tools,runcom/tools,matryer/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,josharian/tools,syreclabs/go-tools,cw2018/tools,cmars/tools,argandas/tools,sshetty10/tools,josharian/tools,josharian/tools,dmitris/tools,dmitris/tools,sshetty10/tools,4ad/tools,golang/tools,nicot/tools,maddyonline/tools,magiccao/tools,runcom/tools,argandas/tools,jdhenke/tools,sshetty10/tools,nicot/tools,cw2018/tools,syreclabs/go-tools,matryer/tools,magiccao/tools,cw2018/tools,magiccao/tools,golang/tools,syreclabs/go-tools,glasser/golang-tools,springning/tools