Upgrading plugins was broken for various reasons. There are four
fixes and a minor one:
1. Use a directory name based only on the `Name` of the plugin, not
the source filename, which contains the version. Avoids strange
duplication of the plugin.
2. Use the new directory name for the deletes if it's present, so
that installation and removal happen at that directory level
and we don't leave empty folders laying around. Ensures we
properly remove additional resources in plugins too, not just
the main `.dll` file.
3. Ignore the incoming `target` when installing, and always set
it ourself to the proper directory, which would matter when
reinstalling.
4. Deletes an existing target directory before installing if it
exists. Note that not calling any of the plugin removal code
is intentional; I suspect that would delete configurations
unexpectedly when upgrading which would be annoying. This way,
it just replaces the files and then reloads.
5. (Minor) Added some actual debug messages around the plugin
download section so failures can be more accurately seen.
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