Only load a manifest if it has not been loaded

[ssteinbach]

• During plugin loading, checks to see which source_files have already been loaded before loading a new plugin.
• For pkg_resource plugins that use the function approach rather than the manifest.json approach, generates a source_files path based on the path to the module containing the function.
• Adds unit tests to check duplicate entries.
• Also updates and clarifies some documentation around how the plugin manifests are loaded.

[avrata]

This all looks good to me. I did have two thoughts but in thinking about it more I don't know if they are really worth any changes:

• Any concern with relative vs absolute paths? This code did not try and normalize them before, but with using the file path as a key now, there could be some inadvertent missed collisions if they are intermixed.
• Could/should this all be localized into Manifest.extend()? I don't see an easy way to do this currently though using source_files. (such that people might want to manually call this, but could get into trouble if so)

I still wish I could track down what the "duplicate schema def" issue was and if we should be super strict about never allowing duplicates in any of these internal lists or not.

[ssteinbach]

@avrata I think this function could use a clean up pass and refactor how the manifests aggregate and are loaded. But this adds a check for relative vs absolute paths.

[codecov-commenter]

Codecov Report

Merging #945 (44cfacc) into master (1f92548) will decrease coverage by 0.02%.
The diff coverage is 80.70%.

@@            Coverage Diff             @@
##           master     #945      +/-   ##
==========================================
- Coverage   85.63%   85.61%   -0.03%     
==========================================
  Files         191      191              
  Lines       18160    18204      +44     
  Branches     2061     2071      +10     
==========================================
+ Hits        15552    15585      +33     
- Misses       2085     2088       +3     
- Partials      523      531       +8     

Flag	Coverage Δ
unittests	85.61% <80.70%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_plugin_detection.py	81.57% <69.23%> (-2.55%)	⬇️
tests/test_adapter_plugin.py	88.72% <83.33%> (-0.94%)	⬇️
...-opentimelineio/opentimelineio/plugins/manifest.py	86.56% <84.61%> (-1.77%)	⬇️

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[avrata]

Looks great! Thanks