rustc: Fix shadowing private import with reexport

The reexport didn't switch the privacy, so the reexport was actually considered
private, erroneously failing to resolve imports later on.

Closes #14082
auto merge of #14236 : alexcrichton/rust/issue-14082, r=brson

The reexport didn't switch the privacy, so the reexport was actually considered
private, erroneously failing to resolve imports later on.

Closes #14082