Support PDE DS Plugin classpath additions

[laeubi]

The PDE DS Plugin has a configuration file named org.eclipse.pde.ds.annotations.prefs that looks like this:

classpath=true
dsVersion=V1_3
eclipse.preferences.version=1
enabled=true
generateBundleActivationPolicyLazy=true
path=OSGI-INF
validationErrorLevel=error
validationErrorLevel.missingImplicitUnbindMethod=error

the interesting part is classpath=true what adds the DS annotation classes to the classpath of the current project.

While everything works in PDE, the tycho build fails with:

Access restriction: The type 'Activate' is not API
  (restriction on classpath entry 'somepath/org.eclipse.osgi.services-3.10.200.v20210723-0643.jar')

so it is required to add a package-import to the bundle to make this compile. The DS annotations are all have a retention of RetentionPolicy.CLASS that means they are not available at runtime and thus only needed at compile-time.

We should add support for this to not force people adding useless imports to their bundle.

[laeubi]

This is also backported to 2.7.2