Added plugins to run Checkstyle and generate reports

[ajaychandran]

Added plugins

• CheckstyleModule, fully compliant with Checkstyle CLI
• CheckstyleXsltModule, generates reports by applying XSL Transformations on a Checkstyle output report

Resolves #3332.

[ajaychandran]

Pending
On running mill.contrib.checkstyle.CheckstyleModuleTest.arguments.stdout, the Checkout output report is not printed to the console.

-------------------------------- Running Tests --------------------------------
mill.testkit.UnitTester: [8/8] checkstyle 
mill.testkit.UnitTester: running checkstyle ...
Checkstyle ends with 4 errors.
mill.testkit.UnitTester: checkstyle found 4 violation(s)
+ mill.contrib.checkstyle.CheckstyleModuleTest.arguments.stdout  

I suspect this is an environment issue. Will investigate further.

[ajaychandran]

@lihaoyi Please review.

[lihaoyi]

This should be moved into a T.source task to ensure changes to that upstream folder properly invalidate this downstream target

[ajaychandran]

Remove CheckstyleXsltReport and replace checkstyleXsltReports with

def checkstyleXslt: T[Seq[PathRef]] = T.sources { ??? }

def checkstyleXsltOutput: T[Seq[PathRef]] = T { ??? }

I can submit a PR. Let me know.

[lihaoyi]

no worries, I already opened on #3522

[lihaoyi]

I think this looks good, if you're happy with it I'll merge it. Another CheckStyle PR has been submitted that has also been of great help understanding the solution space (#3472), so I'll pay out 70% of the bounty for this one and 30% of the bounty for the other. @ajaychandran email me your bank transfer details and I'll wire it over

[ajaychandran]

@lihaoyi Thank you for the guidance.