introduce the option `allowEmptyCodegenFiles` to generate all generated files,
even if they are empty.
- also provides the original source files from which the file was generated
in the write file callback
- needed e.g. for G3 when copying over pinto mod names from the original component
to all generated files
use `importAs` from flat modules when writing summaries
- i.e. prevents incorrect entries like @angular/common/common in the .ngsummary.json files.
change interaction between ng and ts to prevent race conditions
- before Angular would rely on TS to first read the file for which we generate files,
and then the generated files. However, this can break easily when we reuse an old program.
don’t generate files for sources that are outside of `rootDir`
(see #19337)
