[MCOMPILER-407] Feature request: In debug mode, print full output of javac (including stdout)

**[Manu Sridharan](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=msridhar)** opened **[MCOMPILER-407](https://issues.apache.org/jira/browse/MCOMPILER-407?redirect=false)** and commented

Certain javac annotation processors and plugins like [The Checker Framework](https://checkerframework.org/) sometimes print useful diagnostics on stdout.  Currently, there is no way to see this output using the Maven Compiler Plugin, even when Maven is run with the -X option.  I suggest that when Maven is in debug mode, the plugin should show as a debug message the full stdout of javac.  This should also work if javac is forked.

If someone points me at the relevant code, I am happy to try to submit a pull request for this change.

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**Affects:** 3.8.1