Fix false positive of AvoidAlias rule for implicit aliasing of Get- commands for the CommandType ExternalScript

[bergmeister]

PR Summary

Fixes #1369
Unfortunately this issue could only be reproduced when PSSA is hosted within PSES where the root of the workspace becomes the working directory of the runspace pool, which causes Get-Command ..\PathToScript.ps1 to return a command type of ExternalScript, which then makes it into the command cache and causes the false positve.
The fix is to search only for cmdlets/functions where the implicit Get- aliasing applies.

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[JamesWTruher]

This looks great - do we need any additional validation, or is what we have sufficient?

[bergmeister]

The manual end to end test (in vs code) is all I could as somehow this happens only when PSSA is hosted in the vs code extension where those command info cache entries got populated.