The source parsing logic was heavily directed towards Github compatible
source URIs, however if we want to support more cases, we need to make
sure we are able to specify those URIs, and to load plugins installed
from those sources.
Right now, since the getters available are only github.com, we will not
support remotely instlling plugins from sources other than github.com,
with the same set of constraints as before. However, we do support now
installing from a local plugin binary to any kind of source, and we
support loading them, including if a template wants this plugin
installed locally with version constraints.