Workfile Templates: Fix ayon core refactor usage of correct placeholder data

[BigRoy]

Changelog Description

Fix workfile templates since ayon core refactoring family -> product_type

Additional info

Fixes:

# ========
# 'family'
# ========
# Traceback (most recent call last):
#   File "E:\dev\ayon-core\client\ayon_core\pipeline\workfile\workfile_template_builder.py", line 716, in populate_scene_placeholders
#     placeholder_plugin.populate_placeholder(placeholder)
#   File "E:\dev\ayon-core\client\ayon_core\hosts\maya\api\workfile_template_builder.py", line 246, in populate_placeholder
#     self.populate_load_placeholder(placeholder)
#   File "E:\dev\ayon-core\client\ayon_core\pipeline\workfile\workfile_template_builder.py", line 1577, in populate_load_placeholder
#     for repre_entity in self._get_representations(placeholder)
#   File "E:\dev\ayon-core\client\ayon_core\pipeline\workfile\workfile_template_builder.py", line 1471, in _get_representations
#     product_type = placeholder.data["family"]
KeyError: 'family'

Fix is also present in other larger PRs, but they make take some time to get along:

• Workfile Templates: Implement registering and discovering of PlaceholderPlugin #425
• Maya: Load published workfile as template #330
• Workfile Templates tweaks: plugin discovery, event system, maya fixes #327
• Chore: Workfile template builder context usage fix #416

This implements the fix in isolation.

Testing notes:

1. This should fix workfile templates in ayon core.

[iLLiCiTiT]

Resolved in #416