Add ModuleScope as a query

This is a first step towards queryifing completion and resolve.

Some code currently duplicates ra_editor: the plan is to move all
completion from ra_editor, but it'll take more than one commit.

crates/ra_editor/src/completion.rs

@@ -1,3 +1,5 @@
+/// FIXME: move completion from ra_editor to ra_analysis
+
 use rustc_hash::{FxHashMap, FxHashSet};
 
 use ra_syntax::{

crates/ra_editor/src/scope/mod_scope.rs

@@ -1,3 +1,9 @@
+/// FIXME: this is now moved to ra_analysis::descriptors::module::scope.
+///
+/// Current copy will be deleted as soon as we move the rest of the completion
+/// to the analyezer.
+
+
 use ra_syntax::{
     ast::{self, AstChildren},
     AstNode, SmolStr, SyntaxNode, SyntaxNodeRef,