"Resetting cached network information" printed for new PackageDependencyReporter and PackageFunctionReporter

[jayqi]

Issue: Initializing new PackageDependencyReporter and PackageFunctionReporter and using set_package will result in an unintuitive "Resetting cached network information..." logger statement to be printed.

To reproduce:

> myDepReporter <- PackageDependencyReporter$new()
> myDepReporter$set_package('baseballstats')
INFO [2018-03-25 20:22:31] Resetting cached network information...

and

> myFuncReporter <- PackageFunctionReporter$new()
> myFuncReporter$set_package('baseballstats')
INFO [2018-03-25 20:22:48] Resetting cached network information...

The reason this happens is because the reset_cache method of AbstractPackageReporter is checking whether the cache is null. Both PackageFunctionReporter and PackageDependencyReporter start with cache as a list of several variables with null values.

The more complicated and less flexible way to fix this is to have reset_cache check if cache is a list, and then check whether each value in the list is null. This will not work well if we ever end up having cached fields that initialize as a non-null value.

Another implementation that addresses this is to have cache initialized as null with a new reporter instance, and a template default_cache that the cache gets set/reset to. See implementation in older version here and here: