Some internal and language-level changes to name resolution.

Internally, name resolution will be split into two parts - import resolution and
name lookup. Import resolution is moved forward in time to happen in the same
phase as parsing and macro expansion. Name lookup remains where name resolution
currently takes place (that may change in the future, but is outside the scope
of this RFC). However, name lookup can be done earlier if required (importantly
it can be done during macro expansion to allow using the module system for
macros, also outside the scope of this RFC). Import resolution will use a new
algorithm.

The observable effects of this RFC (i.e., language changes) are some increased
flexibility in the name resolution rules, especially around globs and shadowing.

HT @jseyfried