WirePlugin code gen task doesn't declare outputs correctly

[ChrisCraik]

The code gen task in the wire plugin adds the generated code directory to sourcesets, but doesn't declare the directory's files as outputs via fileProvider.

Noticed this as I started using Wire in KMP project in AndroidX where we have checks for this sort of pattern, and I got an error about ':benchmark:benchmark-traceprocessor:generateCommonMainProtos' not being explicitly depended on by ':benchmark:benchmark-traceprocessor:multiplatformSourceJar' (which is a task that depends on all the source directories).

[oldergod]

I'll see next week if we can fix this easily. Thanks for reporting.