* For duplicate source files of the same package, make one redirect to the other
* Add reuseProgramStructure tests
* Copy `sourceFileToPackageId` and `isSourceFileTargetOfRedirect` only if we completely reuse old structure
* Use fallthrough instead of early exit from loop
* Use a set to efficiently detect duplicate package names
* Move map setting outside of createRedirectSourceFile
* Correctly handle seenPackageNames set
* sourceFileToPackageId -> sourceFileToPackageName
* Renames
* Respond to PR comments
* Fix bug where `oldSourceFile !== newSourceFile` because oldSourceFile was a redirect
* Clean up redirectInfo
* Respond to PR comments
