Consistency fix (breaking): LibraryItemKey is replaced by CollectionKey and ContainerKey

[bradenmacdonald]

The opaque keys used for items in libraries are:

• LibraryUsageLocatorV2 ( a UsageKey - the superclass usagekey is for XBlocks anywhere). Well established.
• LibraryCollectionLocator (a LibraryItemKey - a new superclass for non-XBlock items in libraries). New-ish.
• LibraryContainerLocator (a LibraryItemKey - a new superclass for non-XBlock items in libraries). New.

Thinking about this, it's going to be very confusing that we use the term "item" for anything in a library but XBlock keys in libraries (LibraryUsageLocatorV2) are not LibraryItemKeys. It's also weird that the base UsageKey is for XBlocks anywhere but the base LibraryItemKey is for two different types of things and is specific to libraries.

This PR clears up this and makes it consistent via a slight breaking change to the following:

• LibraryUsageLocatorV2 ( a UsageKey ) - no change
• LibraryCollectionLocator (change to a CollectionKey , a new base class)
• LibraryContainerLocator (change to a ContainerKey, a new base class for containers in libraries and - in the future - in courses)
• (Future: also we'd have a LibraryAssetKey which subclasses the existing AssetKey for static assets in libraries).

This is a breaking change but won't affect much code and I think it's important to avoid confusion and mess in the future. Here are the corresponding changes to content_libraries and related apps in edx-platform: openedx/openedx-platform#36588

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[bradenmacdonald]

This refactor allows us to get rid of this messy import of a locator (subclass) in the keys (parent class) file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.00%. Comparing base (15ae68c) to head (ce976fb).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   93.84%   94.00%   +0.15%     
==========================================
  Files          31       31              
  Lines        3007     3036      +29     
  Branches      192      191       -1     
==========================================
+ Hits         2822     2854      +32     
+ Misses        159      156       -3     
  Partials       26       26              

Flag	Coverage Δ
unittests	94.00% <100.00%> (+0.15%)	⬆️

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