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

[sonarqubecloud]

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 80.35%. Comparing base (51b0a00) to head (9212888).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1662      +/-   ##
============================================
+ Coverage     80.04%   80.35%   +0.30%     
- Complexity        0       56      +56     
============================================
  Files            73       76       +3     
  Lines          5438     5605     +167     
  Branches       1101     1147      +46     
============================================
+ Hits           4353     4504     +151     
+ Misses          683      680       -3     
- Partials        402      421      +19     

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