Introduce ModuleId

Previously, module was synonym with a file, and so a module could have
had several parents. This commit introduces a separate module concept,
such that each module has only one parent, but a single file can
correspond to different modules.

crates/ra_analysis/src/lib.rs

@@ -13,7 +13,6 @@ extern crate salsa;
 mod db;
 mod descriptors;
 mod imp;
-mod module_map;
 mod roots;
 mod symbol_index;