Annotation combiner: use ClassLoader#getResources

It is much more robust to use ClassLoader's getResources() method than
trying to iterate the class path elements extracted from the
java.class.path property, especially when running via exec-maven-plugin
(which does not have the relevant .jar files in the system class path).

While at it, look for both modern and legacy annotations, but return only
file names prefixed with the modern prefix in getAnnotationFiles().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
AbstractIndexWriter: support adapting read-back annotations

We will switch the annotation combiner soon to use the existing infrastructure to read the annotation indexes so that both legacy and modern annotation indexes are read correctly.

Of course, we want to write the combined index out again, so let's handle the case where the annotations come from our own ...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Revert "AbstractIndexWriter: support adapting read-back annotations"

This reverts commit a10f7b1c257434bbbb7e4a49d7eae5520af712a2.
CombineAnnotations: support combining legacy annotation indexes, too

Instead of reading all the JSON-formatted indexes ourselves, we can simply
ask the Index class to get all of them for us, including the legacy
annotation indexes, and then write them out using the existing
infrastructure.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CombineAnnotations: make the output directory overrideable

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CombineAnnotations: use the current Thread's context class loader

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move CombineAnnotations into the appropriate package

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>