Add a plugins command to list installed plugins and check updates

Add a "plugins" command to the platform-neutral mvt command. "plugins
list" shows every installed plugin package with its version, where it was
installed from, how many forensic modules it contributes and which
commands it adds. "plugins check-updates" checks for updates immediately,
without waiting for the automatic check, and prints the command which
upgrades a plugin instead of installing anything.

It lives on mvt only. The packages it lists extend mvt-ios and mvt-android
too, but auditing them is not the job of a command which analyses one
platform, and the two platform CLIs should not carry commands which are
not about an acquisition.

The command is registered as a built-in, before any external command, so
that an installed package cannot replace this audit surface.

docs/development/index.md

@@ -214,6 +214,9 @@ module came from:
 - When a command runs with an `--output` folder, the `command.log` file
   records one line per module source with the source's version or hash and
   the list of modules loaded from it.
+- `mvt plugins list` lists the installed packages, where each of them
+  was installed from and how many modules it contributes, see
+  [Managing Plugins](plugins.md).
 
 ## Profiling