Adds conda_dependencies to GitSource

[jrbourbeau]

This PR adds a conda_dependencies property to the GitSource class for installing source project-related dependencies into the test environment. This is similar to how dependencies are installed for the GitTarget class.

Motivating example

When setting up a GitSource for Dask and a GitTarget for XArray, the install_command property for Dask could be either pip install -e . to just install Dask, or pip install -e .[complete] to install Dask and its dependencies from PyPI. However, it would be nice to not mix pip installed dependencies from Dask and conda installed dependencies from XArray. Moreover, other projects may have difficult to install dependencies so conda installing deps would be more than just a preference.

What do you think about this approach @esc?

[esc]

LGMT! Thanks for the patch!