Packages the native assets for iOS and MacOS in frameworks.

Issue:

* https://github.com/flutter/flutter/issues/140544
* https://github.com/flutter/flutter/issues/129757

## Details

* [x] This packages dylibs from the native assets feature in frameworks. It packages every dylib in a separate framework.
* [x] The dylib name is updated to use `@rpath` instead of `@executable_path`.
* [x] The dylibs for flutter-tester are no longer modified to change the install name. (Previously it was wrongly updating the install name to the location the dylib would have once deployed in an app.)
* [x] Use symlinking on MacOS.