A linter warning for module versions that are referenced but not found in BRM

**Is your feature request related to a problem? Please describe.**
This is a feature request for the VSCode extension. In AVM, we're deprecating modules from time to time and it would be great if we could make users aware in some shape of for that this is the case.

**Describe the solution you'd like**
I'd suggest to add a Linter Rule that warns users that a currently referenced module version is **not** found in the 'known' versions. This could be for a few reasons
1. The module is not yet published
2. The cache is not updated
3. The module was deprecated

As all are viable, it would make sense to simply warn the user that a module cannot be found but may still be viable - maybe with some pointer to the above listed reasons.
For somebody who's already using a module for some time, showing this warning would imply it may have been deprecated (as its not part of the module index anymore).

It's not perfect and a more direct way of warning a user of an entire's module deprecation would be nicer - but it's comparably simple and would not require any added systems.

cc: @jtracey93 , @matebarabas

Start by locating the VSCode extension's existing linter rules and the BRM lookup for known module versions. Trace how referenced module versions are resolved, then define the warning for versions absent from that lookup. Done means an appropriate warning appears while preserving the possibility that the module is unpublished, cached incompletely, or deprecated.

Written by the indexing model from the issue text.