Bug 37146: Prevent path traversal by validating input

Bug 37146: Prevent path traversal by validating input

This patch validates the plugin_name passed to plugin_launcher.pl
against the base path containing the "value_builder" directory.

Test plan:
0. Apply the patch
1. koha-plack --reload kohadev
2. Go to http://localhost:8081/cgi-bin/koha/cataloguing/addbiblio.pl?biblionumber=29
3. Check that the tag editor for leader still works
4. Go to http://localhost:8081/cgi-bin/koha/cataloguing/additem.pl?biblionumber=29
5. Check that the pluginf or "Date acquired" still works

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>