[WIP] testing subcommands with pluggy

[skshetry]

How to test:

The plugins directory contains a reference dvc hello subcommand.

1. check out this PR, and chdir to root of the repo.
2. pip install -e "." if required.
3. Invoke dvc as:

 dvc --plugins plugins [hello]

You can add more files to this directory while testing, or in a new separate directory. The only thing that you need to implement in the file is register_command() as done in hello.py.

If you want to test out with a real project, you can also try by creating a simple python project. Here's a simple script to run previous hello example as a package:

mkdir dvc-hello
cp plugins/hello.py dvc-hello
pushd dvc-hello
cat << EOF > setup.py
from setuptools import setup
setup(name="dvc-hello", entry_points={"dvc": ["hello = hello"]}, py_modules=["hello"])
EOF
pip install -e "."
popd
dvc --help
dvc hello