changed goroot subdir detection logic slightly to avoid situation where ...

...GOPATH is a string subset of GOROOT but not actually a child directory

My GOROOT was /home/gfm/go, my GOPATH was /home/gfm/gocode, in a straight prefix comparison without taking the directory separator into account /home/gfm/gocode was being discarded as a srcDir causing massive confusion on my first run of godev (since all I could see was the GOROOT contents and not my GOPATH contents).