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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.38%. Comparing base (6067f35) to head (e370e2d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #439   +/-   ##
=======================================
  Coverage   93.38%   93.38%           
=======================================
  Files           2        2           
  Lines         121      121           
  Branches       25       25           
=======================================
  Hits          113      113           
  Misses          8        8           

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