Fix unused module name warning

[jtdowney]

This fixes an incorrect unused imported module warning when one imports local name matches another imports full module name.

For example

import wibble/wobble
import wobble as my_wobble

pub fn main() {
  wobble.hello()
  my_wobble.hello()
}

Both imports are used, but the compiler reported wibble/wobble as unused.

The type checker resolved wobble.hello() to wibble/wobble correctly. The reference tracker then recorded the use by looking up the string wobble again.

That lookup accepted two different kinds of names:

• A local module name or alias, such as wobble or my_wobble
• A full module name, such as wibble/wobble or wobble

Because the second import's full name was wobble, the reference tracker attached the use to the wrong import. Type checking and generated code were correct. Only the usage graph and the warning were wrong.

I also noticed and fixed a similar but different bug related to an import with a discard alias.

Closes #5975

• The changes in this PR have been discussed beforehand in an issue
• The issue for this PR has been linked
• Tests have been added for new behaviour
• The changelog has been updated for any user-facing changes

[lpil]

Fab, thank you! One tiny note inline