rustc_resolve: Do not allow mods to shadow types

[alexcrichton]

This commit modifies resolve to prevent conflicts with typedef names in the same
method that conflits are prevented with enum names. This is a breaking change
due to the differing semantics in resolve, and any errors generated on behalf of
this change require that a conflicting typedef, module, or structure to be
renamed so they do not conflict.

[breaking-change]
Closes #6936

[rust-highfive]

r? @pnkfelix

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[pnkfelix]

The switches e.g. from NodeSet::new() to NodeSet() did make me wonder if there's something wrong overall with either this "new" rule or our conventions (which say one is supposed to eschew fn TypeName() preferring impl TypeName { fn new() -> TypeName { ... } }, which is not compatible with type TypeName = ... definitions).

But overall I think the rule is the right direction, and I would be happy to revise our conventions to say that fn TypeName() is fine. (Kill the naming convention lints!)

[pnkfelix]

@bors: r+ 3121c04

[bors]

⌛ Testing commit 3121c04 with merge cda3490...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt