Implement a legacy index reader

Our primary concern is SciJava-common; To maintain backwards
compatibility, we want to support reading annotation indexes created by
SezPoz, just in case one class path component was not switched to the
new annotation indexer yet.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! Add the Index class to access the annotation indexes
Index: read legacy annotation indexes transparently

We fall back to using legacy annotation indexes now in case no modern
annotation index was found.

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