Fixed out-of-GOPATH parsing
Lots of nice things before v2:
- Add support for local go module outside of GOPATH. It only works well if "remote" and "local" paths match.
- Fix panicweb on macOS.
- Wrap errors where applicable on go 1.13 and later.
- Ran staticcheck and fixed all warnings.
- Fixed tests on 32 bits when -race is not supported.
- Many internal refactoring in preparation for v2.