Implement a suggested fix to canonicalize (i.e., merge and sort) Clos…

…ure imports (i.e., goog.require, goog.requireType and goog.forwardDeclare) according to the style guide.

The bulk of the logic is in a new RequiresFixer class, which is intended to be reused in CheckRequiresAndProvidesSorted to determine whether a fix is required using the same logic. This will be done in a followup refactor.

As a result, CheckRequiresAndProvidesSorted and ErrorToFixMapper currently disagree on whether a fix is necessary, which requires some tests to be checked in disabled. However, they have been observed to pass in the fully refactored version.

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