Special-case OME's artifacts

They should all go into the jars/bio-formats/ directory because they
clash with SCIFIO's file names.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
install-artifact: DWIM camel-cased properties

It is all-too-easy to mistake the error message to imply that the
property named "imagejDirectoryProperty" should be set. To prevent
confusion when said property *is* set (but imagej.app.directory was
expected to be set), let's just understand what the user wanted.

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