Added Checkstyle plugin

[ajaychandran]

Resolves #3332.

The implementation differs significantly from the SBT counterpart but I feel this approach is idiomatic.

The basic functionality is ready but there is some more work (mostly documentation) to de done.
Could someone take a look and provide feedback?

[ajaychandran]

@lihaoyi
Is there a way to detect the root folder in a project? I am thinking a use-case where the Checkstyle configuration needs to be shared across modules. This is for documentation.

[ajaychandran]

@lihaoyi
I botched a merge. Should I resubmit this in a new PR?

[lihaoyi]

@ajaychandran whatever you prefer. You can also make a new branch locally force push it to the existing PR branch on github

[ajaychandran]

@lihaoyi
A usecase like a CI build would require the build to fail if Checkstyle errors were found.
I will add a task that checks the error count. Is there a naming convention for such tasks?

[lihaoyi]

@ajaychandran we don't have a convention unfortunately; the current names we have used are

./mill -i mill.scalalib.scalafmt.ScalafmtModule/checkFormatAll __.sources + __.mimaReportBinaryIssues + __.fix --check

checkCheckStyle would be a bit weird. I'd say come up with a name that makes sense yourself and we can normalize all these things later

[ajaychandran]

@lihaoyi Please review.

Notable changes:

• checkstyle is now a command that accepts --check and --stdout flags. It also accepts a list of files/folders to be processed.
• The exit code byte conversion was removed.

One issue is that the --stdout flag is not working.
The output report does not show up on the console. The program only prints the number of violations.

[ajaychandran]

@lihaoyi
I think the original design with 2 plugins was cleaner.
If you have no objections, I will close this PR and submit a new one.

[lihaoyi]

go for it