Calling tasks within tasks

[kuwv]

@bitprophet I know this one has been on the back burner for a while. What I wanted to do was see if what I had in mind would work here.

The way I see these tasks would need to be done:

1. Add NamespaceContext to wrap task callables from another namespace.
2. Convert Collection module into a context manager to provide NamespaceContext
3. Integrate new import system into Collection context manager (after this merge: refactor: replace imp with importlib #919)
4. Append results into source context.

Result should look something like:

example.py

from invoke import task


@task
def do_something(ctx, var: str) -> None:
    ctx.run(f"echo {var}")

namespaced.py

from invoke import Collection, task


@task
def doing_something(ctx, var: str) -> None:
    with Collection(ctx, '.example') as nsctx:
        nsctx.do_something('var')