Separate glob imports from item imports

[jjcnn]

Single-item imports shadow glob imports with the same name:

// a.sw
struct Foo = { ... }

// b.sw
struct Foo = { ... }

// c.sw
use a::*;  // Foo is now in scope and refers to a::Foo
use b::Foo; // This is legal, and shadows the previous import so that Foo now refers to b::Foo

We currently represent all imported names in a single map Items::use_synonyms (sway-core/src/semantic_analysis/namespace/lexical_scope.rs), and shadowing is handled when that map is populated.

The code would be clearer if item imports and glob imports lived in separate maps use_glob_synonyms and use_item_synonyms, with the shadowing implemented in the name resolution logic.