Desugar HIR use and extern crate delcarations

[philberty]

These declarations don't have any meaning in the HIR as these statements are used during name resolution which contain AST::SimplePaths which can be fully resolved at that stage.

[philberty]

bors r+

[bors[bot]]

Build failed (retrying...):

• build-and-check

[bjorn3]

pub use still has a meaning, right? It needs to be included in the crate metadata.

[bors[bot]]

Build failed:

• build-and-check

[philberty]

Darn thats a good point @bjorn3 i will look at this again in a few weeks.

[philberty]

I'm going to close this PR and just desugar the ExternBlock out since it doesnt seem to have any effect