[Feature request] Allow generating docs for non-exported files under lib/src

[GroovinChip]

At work I develop an internal plugin that only needs to export one file from its lib/src directory. This unfortunately means that I cannot generate documentation for everything else in lib/src. It would be nice to have an option available in dartdoc_options.yaml that allows this.

[VivienDebio]

I find myself in the same situation. It will indeed be very nice to be able to generate documentation for file in lib/src for both regulatory compliance and maintenance purpose. Especially since the documentation generated is good. In my opinion documentation is nearly as important for the private as for the public part of a package.