fix(plugins): resolve plugin paths using directory tree

[ne-cdietsch]

Summary

• Fixes a bug where NCPA could run the wrong plugin when the same filename existed in both the plugins root directory and a subdirectory (e.g. plugins/get_script_dir.ps1 vs plugins/old/get_script_dir.ps1). Closes NCPA resolves wrong plugin file when duplicate filename exists in a subdirectory #1423. Closes Plugins folder - files in subdirectories have higher precedence #1257
• Builds a directory tree for plugin discovery instead of a flat filename map, so plugins are resolved by their full relative path. Closes The plugins directory is flat - no way to specify a subdir #577.
• Adds support for running plugins in subdirectories via path (e.g. plugins/old/check_disk.sh) and lists them in the plugin inventory. Closes multiple plugin_path #811.
• Adds unit tests covering duplicate filename resolution, subdirectory path access, and plugin listing.

Test plan

• Run python -m unittest test_pluginnodes -v from the test/ directory
• Place get_script_dir.ps1 (or .sh on Linux) in the plugins root and confirm check_ncpa.py -M 'plugins/get_script_dir.ps1' reports the root directory
• Copy the same file to plugins/old/ and confirm the same command still runs the root copy
• Confirm plugins/old/get_script_dir.ps1 runs the subdirectory copy when requested explicitly
• Confirm /api/plugins lists both root and subdirectory plugins with relative paths

[ne-cdietsch]

Possible breaking changes

This fix changes how plugins are discovered and resolved. Most existing checks should keep working, but some may need updates.

Duplicate filenames (most likely impact)

Previously, if the same filename existed in the plugins root and a subdirectory (e.g. plugins/check_disk.sh and plugins/old/check_disk.sh), NCPA kept only one entry and the subdirectory copy could overwrite the root copy. A request to plugins/check_disk.sh might have been running the subdirectory version.

After this change:

plugins/check_disk.sh always runs the root-level file
plugins/old/check_disk.sh runs the subdirectory file

Action required: If a check was intentionally using the subdirectory copy while calling plugins/ (without the subdirectory path), update it to the full relative path, e.g. plugins/old/check_disk.sh.

Checks that target a root-level plugin and have no duplicate filename elsewhere are unaffected.

Checks using subdirectory plugins

If you were running a plugin that lives in a subdirectory (e.g. plugins/old/check_disk.sh) by calling it without the subdirectory path — for example:

check_ncpa.py -H server -t TOKEN -M 'plugins/check_disk.sh'
—that may have worked before only because NCPA indexed plugins by filename alone.

With this fix, plugins/check_disk.sh always runs the root-level file. To run the subdirectory copy, update the check to include the full relative path:

check_ncpa.py -H server -t TOKEN -M 'plugins/old/check_disk.sh'
The same applies to passive checks, NCPA config, or any other reference that used only the basename for a plugin in a subdirectory.

Plugin inventory API

GET /api/plugins now lists subdirectory plugins with their relative path (e.g. old/check_disk.sh), not only root-level basenames. Anything that parses this response and assumes every entry is a root-level filename may need to be updated.

What does not require changes

Checks using plugins/ for a plugin that exists only at the root level
Checks already using an explicit subdirectory path (these may start working where they previously failed)