Warning due to missing explicit constructor in generated classes

[gtache]

When compiling with all the warnings enabled on JDK16+ while using JPMS, we get a bunch of warnings such as :
[WARNING] /abc/target/generated-sources/annotations/abc/RemoteModule_ProvidesObjectMapperFactory.java:[23,14] class pkg.RemoteModule_ProvidesObjectMapperFactory in exported package pkg declares no explicit constructors, thereby exposing a default constructor to clients of module module
[WARNING] /abc/target/generated-sources/annotations/abc/RemoteTranslator_Factory.java:[21,14] class pkg.RemoteTranslator_Factory in exported package pkg declares no explicit constructors, thereby exposing a default constructor to clients of module module
...

I think the easiest way to fix it would be by adding "missing-explicit-ctor" to the existing SuppressWarnings.

[bcorso]

Thanks, looks like the case where we're missing the explicit constructor is here:

https://github.com/google/dagger/blob/master/java/dagger/internal/codegen/writing/FactoryGenerator.java#L146-L152

I think we should just fix this in the code to always generate a constructor, and in fact even the constructors we generate currently are public so we should change them to always be private, so I'll make that change too.

[gtache]

Indeed, if always adding the constructors is not complicated then it's clearly the best solution. Thank you for looking into this!
I'll close my PR then.