Use fine-grained dependencies to eliminate stanza from FlaggedDeps?

@edsko, I'd quite like to know what you think about this.

In a5a823d47b44802e535dbdf3d225da706b78f3bf, 6b77ea23129f8c91d3f1a3dd5d7103b2bbe811b9 and 87a79be901c47d40a972566cbed010feee78e3cc you modified the solver to keep track of which component a dependency came from. However, as far as I can tell, the solver doesn't actually use this information; it just makes sure it gets passed along.

At the same time, there is an orthogonal mechanism, Stanza in FlaggedDep, which lets you conditionalize a dependency tree on whether or not a stanza is enabled. This code substantially predates fine-grained dependencies, and I am now wondering whether or not we need it anymore: why can't we just look at the stored `Component` when building the search tree to decide how to branch? In this way, we automatically get the ability to conditionalize on executables too.