Integrate VirtualEnv Management into Component

[nickjalbert]

Right now, the virtualenv management is completely separated from the Component lifecycle. For example, in the CLI, we:

1. Get the virtualenv
2. Activate the virtualenv
3. Get the Component
4. Run the Component's entry point

However, there's duplicate work going on here because to get the virtualenv (step 1), we need to construct the Component DAG to find all the requirements files associated with the Components. That DAG then gets tossed and then reconstructed in step 3 to get Component.

This implementation was chosen due to some awkwardness around Component instantiation: you need the virtualenv before you import the managed class, and currently Component eagerly imports the managed class in a way that makes it difficult to fully construct the virtualenv beforehand.

However, with the changes proposed in #276, virtualenv management becomes much cleaner as something that happens completely within the Component. This also makes REPL based demos cleaner.

[nickjalbert]

@andyk FYI with the merge of #293, I'm going to work on this.

[andyk]

Excellent! Happy to chat about the design a little more too if that would be helpful.