Switch PkgVersion to use composition

Comparing PkgVersion and Version objects can produce nonsensical
results. For example, equality is not symmetric:

irb(main):002:0> PkgVersion.new("1.0", 0) == Version.new("1.0")
=> false
irb(main):003:0> Version.new("1.0") == PkgVersion.new("1.0", 0)
=> true

Rather than attempt to deal with subclass-superclass equality, let's use
composition and punt on the problem altogether.

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