Fix test failure in testsuite-osgi-deps

This commit fixes both failure of test itself and failure of compiling
and running test.

- When the test was run via 'mvn test', Maven gives karaf-maven-plugin a
  list of class directories instead of OSGi bundles, so that
  karaf-maven-plugin generates incorrect feature.xml.  I added a
  workaround for this specific case to DependencyIT
- When the packaging of project is 'feature', maven-compiler-plugin is
  not run at all.  Added a <plugin/> section so that it's always
  compiled.