When compiling libraries, this feature extracts the minimal information
from the directives/pipes/modules of the library into `.ngsummary.json` files,
so that applications that use this library only need to be recompiled
if one of the summary files change, but not on every change
of the libraries (e.g. one of the templates).
Only works if individual codegen for libraries is enabled,
see the `generateCodeForLibraries: false` option.
Closes#12787
Tobias Bosch