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

[jmaxson-ms]

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 A clear and concise description of what you expected to happen.

Actual behavior A clear and concise description of what actually happens.

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Version information (please complete the following information):

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• WPA version (if applicable):

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