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