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 84.29%. Comparing base (8574240) to head (ead777b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1329   +/-   ##
=========================================
  Coverage     84.29%   84.29%           
  Complexity       42       42           
=========================================
  Files             8        8           
  Lines           605      605           
  Branches        137      137           
=========================================
  Hits            510      510           
  Misses           87       87           
  Partials          8        8           

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