Loading a type multiple times during assembly discovery doesn't cleanup properly

Describe the bug
If a DLL exists in multiple directories, it may be loaded and its types enumerated multiple times during assembly discovery. When this happens, the second time a type is loaded as a ProcessingSourceReference (for example), no references to it are kept, but it is not properly disposed of.

To Reproduce
Steps to reproduce the behavior:

1. Create a plugin that uses feature X/does Y
2. Load the plugin with WPA/the Engine
3. Perform actions ...

Expected behavior
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Actual behavior
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Screenshots
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Version information (please complete the following information):

• SDK version:
• Engine version (if applicable):
• WPA version (if applicable):

Additional context
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