Maximize usage of ImmutableCapabilities to further reduce memory usage and simplify code

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Makes ImmutableCapabilities class thoroughly immutable by verifying immutability of all contents as they are added. This add references to already immutable capabilities, and duplicates non-immutable capabilities as immutable types.

Renames DefaultShadowedCapability to ImmutableShadowedCapability to advertise immutability. Also check that the shadowed capability is immutable upon construction, or make an immutable copy.

The result of this is to make the DefaultArtifactSelector more efficient in two ways. It can defer creation of a fallback capabilities collection until it is actually needed. It can also reuse existing immutable capabilities rather than creating new instances, as the immutable types will never change.

This saves 16MB or so during the large sync scenario testing by creating less ImmutableCapabilities and ImmutableCapability objects which are equivalent.

The top image shows the new results, the bottom shows the old:

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@bot-gradle test this

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@bot-gradle test and merge

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