rustc: Remove a number of mutable fields in cstore

rustc: Remove a number of mutable fields in cstore

This commit started by moving methods from `CrateStore` to queries, but it ended
up necessitating some deeper refactorings to move more items in general to
queries.

Before this commit the *resolver* would walk over the AST and process foreign
modules (`extern { .. }` blocks) and collect `#[link]` annotations. It would
then also process the command line `-l` directives and such. This information
was then stored as precalculated lists in the `CrateStore` object for iterating
over later.

After this, commit, however, this pass no longer happens during resolution but
now instead happens through queries. A query for the linked libraries of a crate
will crawl the crate for `extern` blocks and then process the linkage
annotations at that time.