Spurious "Ignoring unsupported script" debug messages on startup

Upon launching ImageJ2 with `--debug` from the CLI, amongst the usual deluge of messages, we now see:
```
[DEBUG] Ignoring unsupported script: bigwarp_fiji-7.0.7.jar
[DEBUG] Ignoring unsupported script: M_I_P-2.0.1.jar
[DEBUG] Ignoring unsupported script: IO_-4.2.1.jar
[DEBUG] Ignoring unsupported script: SPIM_Registration-5.0.25.jar
...
```
for every JAR file in Fiji's `plugins` folder.

This suggests there is a problem with which folders are being scanned by the SciJava `ScriptFinder`—it is not supposed to be looking in the base `plugins` folder.

Start by tracing SciJava's ScriptFinder startup scan when ImageJ2 is launched with --debug, focusing on why the base plugins folder is included. Reproduce the messages with a Fiji installation containing JAR files, then verify that unsupported JARs in that folder no longer produce debug messages.

Written by the indexing model from the issue text.