checker: include import aliases when checking for import duplicates

[ttytm]

Currently, it's possible to use an import alias as duplicate of another import and an import as duplicate of another import alias.

import json
import x.json2 as json

import x.json2 as json
import json

With the changes in this PR it will result in:

error: `json` was already imported on line 1
    1 | import json
    2 | import x.json2 as json
      |                   ~~~~

error: `x.json2` was already imported as `json` on line 1
    1 | import x.json2 as json
    2 | import json
      |        ~~~~

🤖 Generated by Copilot at bb05497

Add error checks to the checker module to prevent importing modules with duplicate names or aliases. Add test files to verify the new checks.

🤖 Generated by Copilot at bb05497

• Add error checks for importing modules with duplicate names or aliases (link)
  • Create V files with .vv extension that import modules with conflicting names or aliases (link, link)
  • Create output files with .out extension that match the V file names and contain the expected error messages and positions (link, link)

[medvednikov]

Good find!