Place symlink to repo contains Gomfile itself

To enable `import` package from Gomfile's repository using absolute path,
place symlink with same name in original $GOPATH to the Gomfile's repository.

For instance:

- $GOPATH
  - src
    - example.com
      - test
        - Gomfile

The repository "example.com/test" will be available in gondler env at
".gondler/src/example.com/test" as symlink to "$GOPATH/src/example.com/test".:

- example.com
  - test
    - .gondler
      - src
        - example.com
          - test -> ../../.. = example.com/test