Fix bad sad error when a function from a transitive dependency is mocked

[Thevakumar-Luheerathan]

Purpose

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Fixes #42034

Approach

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Samples

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Remarks

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Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (3c3df04) 0.00% compared to head (ef0fc46) 76.69%.
Report is 5 commits behind head on master.

Files	Patch %	Lines
...lerinalang/compiler/tree/BLangTestablePackage.java	50.00%	2 Missing ⚠️
...o2/ballerinalang/compiler/desugar/MockDesugar.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #42091       +/-   ##
=============================================
+ Coverage      0.00%   76.69%   +76.69%     
- Complexity        0    53032    +53032     
=============================================
  Files             9     2883     +2874     
  Lines            35   199939   +199904     
  Branches          0    26025    +26025     
=============================================
+ Hits              0   153344   +153344     
- Misses           35    38138    +38103     
- Partials          0     8457     +8457     

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