Bump to new ImageJ2 package structure

This is needed to make the tests pass: we now explicitly want to use
ImageJ2's new package structure. The switch to the new dependencies also
pointed to a crucial problem with our migration: we still relied on the
old package structure in *parts* of fiji-compat (because we relied on
ij-legacy which is inherently incompatible with ij1-patcher versions >=
0.4.0).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use LegacyInitializer class directly

Now that we have the correct dependencies, we might just as well use the
LegacyInitializer class directly when testing whether the imagej-legacy
dependency can be found in the class path.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a safety net when registering SwitchToModernMode

We just switched to using the SwitchToModernMode class that lives in the
new package name, net.imagej.legacy.

However, we simply cannot rely on the fact that update sites will all be
up-to-date with the bleeding edge all the time. So we *must* deal nicely
with version skew.

So let's fall back to calling SwitchToModernMode in the old package.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a fall-back for the two legacy plugins in case of version skew

When ij-legacy.jar is on the class path and imagej-legacy.jar (which
obsoletes ij-legacy.jar) is not, we still want Fiji to start up
properly, including window title and all.

So let's add back the plugin types with the old package names so that
SciJava Common's annotation indexer picks them up, too, and everything
works whether ij-legacy.jar is available in the class path, or
imagej-legacy.jar, or both.

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