Support installing grumpy-runtime in editable mode

[abitrolly]

pip install --editable . allow to edit source code of Python package and run it without rebuilding the package. grumpy-runtime doesn't support this, and it also takes a long time to rebuild itself.

When runtime is installed in editable mode, the grumpy run command fails with this.

# grumpy -c 'print("hello")'
/tmp/tmptxtN5t__pycache__/gopath/src/__python__/main.go:6:2: cannot find package "__python__/__go__/grumpy" in any of:
	/usr/lib/go-1.13/src/__python__/__go__/grumpy (from $GOROOT)
	/tmp/tmptxtN5t__pycache__/gopath/src/__python__/__go__/grumpy (from $GOPATH)
	/app/grumpy-runtime-src/grumpy_runtime/data/gopath/src/__python__/__go__/grumpy

From #151 (comment)