refactoring source_maps

[sigmundch]

source_maps was a kitchen sink before we had other uses for what we have there. It should probably be refactored as follows:

 - span - should be its own pacakge
 - source_maps parser & builder should stay
 - refactor & printer - should go to code_transformers?

[sigmundch]

+Nathan, since he is already looking into factoring Span into its own package.

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cc @nex3.

[nex3]

Set owner to @nex3.
Added Started label.

[DartBot]

This issue has been moved to dart-lang/tools#1594.