fix[ux]: fix uses error message

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

when a stateless module is accidentally declared as being `used` by a
user, the error message is confusing. update the error message to
clarify `uses` refers to state being used.

Description for the changelog

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.77%. Comparing base (b43ffac) to head (a3bc9b4).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3926      +/-   ##
==========================================
- Coverage   90.94%   90.77%   -0.17%     
==========================================
  Files          95       95              
  Lines       14397    14397              
  Branches     3191     3191              
==========================================
- Hits        13093    13069      -24     
- Misses        904      927      +23     
- Partials      400      401       +1     

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[cyberthirst]

this is also a bit confusing when lib1 isn't used in lib2:

# main
import lib1
import lib2

initializes: lib2
initializes: lib1

@external
def foo():
    lib2.foo()
    lib2.lib1.counter = 1
    
# lib1
counter: uint256

# lib2
import lib1

counter: uint256

@internal
def foo():
    pass

Error compiling: tests/custom/test.vy
vyper.exceptions.ImmutableViolation: Cannot access `lib1` state!

  (hint: add `uses: lib1` or `initializes: lib1` as a top-level statement to your contract)

  contract "tests/custom/test.vy:10", function "foo", line 10:4 
        9     lib2.foo()
  ---> 10     lib2.lib1.counter = 1
  ------------^

could we warn about unused imports?

[pcaversaccio]

could we warn about unused imports?

Fwiw, some time ago (~1 year ago lol), I already opened an issue re this #3272.