Rename the global ij_launcher_jar variable, too

It no longer points to ij-launcher.jar, but to imagej-launcher.jar,
after all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fall back to ij-launcher.jar when imagej-launcher.jar was not found

Hard lesson learnt when migrating the updater from Fiji into ImageJ2:
one always needs more than one fall back layer. Preferably always one
more than one wants to do. It will be needed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fall back to old package name for the ClassLauncher

Second fall back layer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fall back to the old package name when net.imagej.Main was not found

Third fall-back layer.

This still leaves the case when we want to call ImageJ2 with an old
ij-launcher.jar and an old ij-app.jar...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fall back to old package name of main ImageJ (2.0.0) class

When the presence of the now-obsolete ij-app.jar is detected in
conjunction with the absence of imagej.jar, let's assume that we need to
call the main class via the now-obsolete package name.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge remote-tracking branch 'origin/package-name'

This change prepares the launcher for the new package name for
ImageJ2: net.imagej.

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