Ref #927: Improve the support of third party repositories

[essobedo]

Related to #927

Motivation

Some operations such as the automatic download of the Camel Debugger don't consider third-party maven repositories. Moreover, third-party maven repositories are only detected in maven projects, it should be improved to support Gradle projects too.

Modifications:

• Delegates the extraction of third-party maven repositories to MavenArtifactRetrieverContext to make sure that all operations take them into account
• Provides a way to retrieve the list of maven repositories from Gradle project
• Retrieves the list of maven repositories from Maven project using the Maven model instead of using the DOM API
• Removes all DOM-related classes as they are no longer used
• Moves all Gradle-related classes into a gradle package
• Moves all Maven-related classes into a maven package