cmd/api: don’t rely on hardcoded go versions
cmd/api: don’t rely on hardcoded go versions

Instead of requiring that cmd/api/run.go be edited upon each
release to include the next Go version number, look in $GOROOT/api
for files with the prefix go1* and use those instead to perform
API checks.

Change-Id: I5d9407f2bd368ff5e62f487cccdd245641ca9c9b
Reviewed-on: https://go-review.googlesource.com/83355
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,golang/go,codestation/go,codestation/go,josharian/go,golang/go,cloudflare/go,codestation/go,akutz/go,akutz/go,golang/go,CAFxX/go,nawawi/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,Cofyc/go,josharian/go,josharian/go,codestation/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,nawawi/go,nawawi/go,codestation/go,CAFxX/go,akutz/go,Cofyc/go,akutz/go,codestation/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,Cofyc/go,josharian/go,akutz/go,codestation/go,golang/go,golang/go,CAFxX/go,CAFxX/go,Cofyc/go,josharian/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,Cofyc/go,Cofyc/go,codestation/go,josharian/go,golang/go,akutz/go,josharian/go,nawawi/go,CAFxX/go,christopher-henderson/Go