Support certs/ that live within $GOPATH/src

This adds an additional search directory that supports calling
git repositories whose certs/ directory is not at $GOPATH/certs, but
rather at $GOPATH/pkg/path/certs. Subpackages such as pkg/path/server
work as well.
Right.

heh, strings.go:
```go
func Contains(s, substr string) bool {
	return Index(s, substr) >= 0
}
```

But I suppose that's more readable.