cmd/go: do not confuse files for standard library packages
I often create dummy files holding various data named things like 'z'.
If a file (not directory) GOROOT/src/z exists, it confuses cmd/go into
thinking z is a standard library package, which breaks the test
Script/mod_vendor.
This CL fixes internal/goroot to only report that something is a standard
library package when a directory with that name exists, not just a file.