Use the feature resolver for imports and updates.

Also rework and improve the dependency differ to use the fact that the
Dependency type is hashable.

Get rid of crate_tool/src/crates_io.rs because all functionality is now
superseded by crates_io_util.

Test: treehugger, cargo test, ./crate_tool analyze-updates ahash
Change-Id: If7f5a7985afe032f966d13686de791500fafe6fc