fix: adding or removing a file (library) in a patch needlessly reduces link percentage

[eseidel]

I've not written a test case yet, but reading the code I believe this to be true.

Dart has "private keys" for each library (these are not crypto keys, but rather just 1byte unique identifiers for each library (each .dart file) which is used in the naming of private fields (fields that begin with _) to make sure their names are unique globally. That "private key" generation is dependent on compile order (it uses the number of total known libraries at time of key creation), so I suspect that if shorebird users on iOS add or remove files in their patches they will see lower-than-expected link percentages.