[PDE] mark package org.eclipse.m2e.pde as internal

[HannesWell]

Because the only (exported) package of the org.eclipse.m2e.pde is not considered public API (as for example stated in #440 (comment)) we should mark the package as such to make users aware of that fact.
Otherwise downstream-adaptors could accidentally use the classes from that package and assume they are public API, unless they have searched in the depths of the m2e-github repo.

Since org.eclipse.m2e.pde has been released in this form already actually it is already public API, but since this happened not so long ago and adaptors are unlikely (or if it happened there is a high chance that they are still active and can adapt accordingly or complain), I propose to make an exception and mark this package as internal afterwards.

@laeubi, @mickaelistria what do you think?

[laeubi]

If it makes the world better lets merge it :-)

Otherwise downstream-adaptors could accidentally use the classes from that package like public API,

A matter of fact is that people will still use these if they think its useful and simply suppress the warning (as we do in m2e, tycho, ...)

[HannesWell]

If it makes the world better lets merge it :-)

Otherwise downstream-adaptors could accidentally use the classes from that package like public API,

A matter of fact is that people will still use these if they think its useful and simply suppress the warning (as we do in m2e, tycho, ...)

A little bit better, I think :-)
Of course it does not prevent external usage (I admit that I also sometimes use internal API if there is no public equivalent), but at least we tried our best and made down-stream users aware of it, so they (hopefully) know the implications. Unfortunately world is not perfect.