Merge `ModuleTree` and `ModuleItemMap`

[matklad]

Currently, building the tree of modules and resolving use items are separate phases. This is because we don't handle macros yet :-)

WIth macros, a module can be defined by a macro invocation, which is imported by a use path. To handle this, we should merge two things into one.

Specifically, we should create a new struct and query, CrateSymbols, which contains a module tree, a mapping from module to declared/imported symbols and a mapping from macro invocations to the expansion results.

The LowerModule step should work on the raw syntax and should be oblivious of the particular module. Specifically, it shouldn't do macro expansion, while it does it today.

A possible further extension here is to split import into two categories: imports that can affect macro resolution, and other imports. Ideally, we should lazily resolve the second category.

[matklad]

cc @pnkfelix @michaelwoerister, this the main "name resolution/macro expansion" issue in rust-analyzer.

The code which handles this currently is

https://github.com/rust-analyzer/rust-analyzer/blob/12e3b4c70b5ef23b2fdfc197296d483680e125f9/crates/ra_hir/src/module_tree.rs

https://github.com/rust-analyzer/rust-analyzer/blob/12e3b4c70b5ef23b2fdfc197296d483680e125f9/crates/ra_hir/src/nameres.rs

#724 (in-progress PR which adds macro-expansion, outside of the resolution atm)

[matklad]

fixed