Add Maven Checkstyle

[bact]

Add Maven Checkstyle correctness check in CI.

Can run mvn checkstyle:check from command line too.

[goneall]

Running this locally, I'm getting two checkstyle errors on the current codebase:

[ERROR] src\main\java\org\spdx\tools\schema\AbstractOwlRdfConverter.java:[224,94] (coding) EqualsAvoidNull: String literal expressions should be on the left side of an equals comparison.
[ERROR] src\main\java\org\spdx\tools\Verify.java:[232,59] (coding) ModifiedControlVariable: Control variable 'verifyMsg' is modified.

We should fix those before merging this in to avoid CI failures.

[goneall]

Running this locally, I'm getting two checkstyle errors on the current codebase:

[ERROR] src\main\java\org\spdx\tools\schema\AbstractOwlRdfConverter.java:[224,94] (coding) EqualsAvoidNull: String literal expressions should be on the left side of an equals comparison.
[ERROR] src\main\java\org\spdx\tools\Verify.java:[232,59] (coding) ModifiedControlVariable: Control variable 'verifyMsg' is modified.

We should fix those before merging this in to avoid CI failures.

I just noticed the configuration won't cause it to fail on validation, so this should be fine.

@bact - if you could resolve the merge conflict, I'll go ahead and merge it

[bact]

Running this locally, I'm getting two checkstyle errors on the current codebase:

[ERROR] src\main\java\org\spdx\tools\schema\AbstractOwlRdfConverter.java:[224,94] (coding) EqualsAvoidNull: String literal expressions should be on the left side of an equals comparison.
[ERROR] src\main\java\org\spdx\tools\Verify.java:[232,59] (coding) ModifiedControlVariable: Control variable 'verifyMsg' is modified.

Fixed.

Also resolved merge conflicts.

I also set checkstyle fail on violation = true.