parser: fix case of falsely registering imports as used, remove unused imports

[ttytm]

Fixes e.g,

import term as te
import te.ui // alias `te` as part of another import would register the import `term` as used, despite its never being used.

fn main() {}

The change reveals a lot of unused imports that are removed in this PR as a requirement to pass CI.

[JalonSolov]

In this particular case, I'd think it would report both term and term.ui as being unused.

Will that happen with this change?

[ttytm]

There is a test added that correctly recognizes them as not being used

import term as te
import te.ui

vlib/v/checker/tests/unused_import_err.vv:1:8: warning: module 'te (term)' is imported but never used
    1 | import term as te
      |        ~~~~
    2 | import te.ui
vlib/v/checker/tests/unused_import_err.vv:2:8: warning: module 'ui (te.ui)' is imported but never used
    1 | import term as te
    2 | import te.ui
      |        ~~~~~
vlib/v/checker/tests/unused_import_err.vv:2:1: builder error: cannot import module "te.ui" (not found)
    1 | import term as te
    2 | import te.ui
      | ~~~~~~~~~~~~

[spytheman]

Good work.