Replace import iteration with dead code elimination during imports

[udoprog]

This removes the verify_imports stage of compilation by integrating imports into the indexing/build system.

It's still kinda iffily integrated, in that the get_imports handling is done separately from query_meta. This is because imports can have cycles, and query_meta just doesn't have the necessary plumbing to deal with it. There's also a fair amount of metadata that would have to be awkwardly propagated (like import chains used for diagnostics), so it might not be worth it.

This is what cyclic import checking looks like right now: