SWARM-888: Gradle Plugin Hardcoded Modules resource directory

[AdlerFleurant]

Motivation

org.wildfly.swarm.plugin.gradle.SwarmExtension contains the below line:
this.moduleDirs.add(new File(this.project.getBuildDir(), "resources/main/modules"));

The output location is hardcoded. Gradle has the ability to modify the
resource output directory and this code will silently ignore modules
whenever the resource output directory is changed through Gradle.

Modifications

The hardcoded line was changed to utilize Gradle Main SourceSet output
directory. The solution still expects a "modules" directory under the output resource
directory.

Result

Changing Gradle resource output directory will no longer prevent the
loading of the modules.

• Have you followed the guidelines in our Contributing document?
• Have you created a JIRA and used it in the commit message?
• Have you checked to ensure there aren't other open Pull Requests for the same issue?
• Have you built the project locally prior to submission with mvn clean install?

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[thorntail-ci]

Can one of the admins verify this patch?

[thorntail-ci]

Can one of the admins verify this patch?

[kenfinnigan]

ok to test