With search-remote-licenses enabled, every module name went to the proxy,
including toolchain binaries whose main module is synthesized from the
package path, such as cmd/cgo. The proxy answers 404, and the direct
fallback then treats the path as a repository host, producing requests
like https://cmd/cgo/info/refs?service=git-upload-pack.
Reuse isStandardImportPath, already in this package, to skip the remote
search for paths whose first element carries no dot. Those are never
publishable module paths, so neither a proxy nor a repository can resolve
them.
Fixes#3149
Signed-off-by: Luan Taraschi <130802253+luantaraschi@users.noreply.github.com>
