Fixes for Checkstyle's breaking release versions 8.1 and 8.2

[thiloshon]

Purpose

Checkstyle had breaking releases (8.1 and 8.2) which made it impossible to use WSO2 config XML in new versions of Checkstyle. This PR contains fixes for two such issues which will enable using the config in newer versions of Checkstyle.

Resolves #4

Approach

Fix for breaking feature of Release 8.1

Making SuppressionCommentFilter and SuppressWithNearbyCommentFilter children of TreeWalker.
Refer:

1. http://checkstyle.sourceforge.net/releasenotes.html#Release_8.1
2. "SuppressionCommentFilter is not allowed as a child in Checker" jshiell/checkstyle-idea#341
3. Make SuppressionCommentFilter and SuppressWithNearbyCommentFilter children of TreeWalker checkstyle/checkstyle#4714

Fix for breaking feature of Release 8.2

Removing FileContentsHolder module as FileContents object is available for filters on TreeWalker in TreeWalkerAudit Event

1. http://checkstyle.sourceforge.net/releasenotes.html#Release_8.2
2. remove FileContentsHolder module as FileContents object is available for filters on TreeWalker in TreeWalkerAudit Event  checkstyle/checkstyle#3573
3. Cannot use checkstyle version 8.2: Unable to instantiate 'FileContentsHolder' class checkstyle/checkstyle#5132

[maheshika]

staled