Make README.md as the default package documentation

[NipunaRanasinghe]

This PR includes the following changes:

• Rename Module.md to README.md in the default module and submodules
• Remove Package.md from the default module

Related to ballerina-platform/ballerina-library#7404

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.82%. Comparing base (5e996a5) to head (7d5acd2).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2226      +/-   ##
============================================
- Coverage     80.41%   79.82%   -0.59%     
- Complexity        0      678     +678     
============================================
  Files           366      417      +51     
  Lines         20706    23073    +2367     
  Branches       3285     3744     +459     
============================================
+ Hits          16650    18419    +1769     
- Misses         3122     3524     +402     
- Partials        934     1130     +196     

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