Remove usage of internal Asciidoctorj DirectoryWalker

[abelsromero]

What is this issue about?

• Bug report
• Feature request
• Question
• Refactor/Improvement

Description
Currently, we use org.asciidoctor.jruby.AsciiDocDirectoryWalker and org.asciidoctor.jruby.DirectoryWalker to obtain AsciiDoc sources from the paths. But his classes are not part of the asciidoctorj-api but the jruby implementation, this breaks encapsulation and prevents the plugin from working with other AsciidoctorJ implementations using the service loader.
We should replace this by some agnostic piece with same functionality, for example FileUtils.listFiles.

Reference:

asciidoctor-maven-plugin/src/main/java/org/asciidoctor/maven/AsciidoctorMojo.java

Lines 439 to 450 in 2a79b16

	protected List<File> calculateSourceFiles(File sourceDirectory) {
	if (sourceDocumentName != null)
	return Arrays.asList(new File(sourceDirectory, sourceDocumentName));
	// Both DirectoryWalkers filter out internal sources and path (_ prefix)
	final String sourceDirectoryAbsolutePath = sourceDirectory.getAbsolutePath();
	final DirectoryWalker directoryWalker = sourceDocumentExtensions.isEmpty()
	? new AsciiDocDirectoryWalker(sourceDirectoryAbsolutePath)
	: new CustomExtensionDirectoryWalker(sourceDirectoryAbsolutePath, sourceDocumentExtensions);
	return directoryWalker.scan();
	}

PD: there are other jruby specifics being used, but we can treat them in another issue.

[abelsromero]

Since this is internal change, there should be no need for new tests.

[stdll]

Hi! Sorry for spamming this issue. I've looked into the issue and came up with a PR. If you have feedback, please let me know.

[abelsromero]

Hi! Sorry for spamming this issue.

No problem 👍 My only suggestion is if you have a long running PR, just mark as "Draft" so people is aware is not finished. But even that's opinionated and some ppl prefer not getting the PR untill is finished 🤷

I can't promise I can look into the issue today, but I'll try.