Improper isStdlib() path check using strings.HasPrefix()

[jingyuanliang]

I have a setup where my GOROOT is /usr/local/go and source files are stored under /usr/local/google. go-licenses doesn't output anything in this case. I've traced it down and it's due to

go-licenses/licenses/library.go

Line 289 in 13c8b64

return strings.HasPrefix(pkg.GoFiles[0], build.Default.GOROOT)

where build.Default.GOROOT got /usr/local/go and pkg.GoFiles[0] got /usr/local/google/... so it thinks everything is stdlib, then skips all of them.

[Bobgy]

looks like we can just check prefix /usr/local/go/ instead

[Bobgy]

Duplicate of #38