Don't require an Ant target definition for each Maven goal

[berndv]

For builds with custom ant targets it is currently required to define a corresponding ant target in the <project>/externalBuild/customTargets.xml file for each maven phase. This is inconvenient, error prone and leads to a lot of unnecessary log messages from empty ant calls during the build process.

The idea is to scan the customTargets.xml for targets named like maven phases and only to execute those during the build.