6158953b

By: Michael Lynch <git@mtlynch.io>

Unexport uploads' proof-carrying Processable types

ProcessableImage/ProcessableVideo and their constructors had no production
callers outside uploads once ProcessImage/ProcessVideo became the package's
only entry points; test files can't use an in-package export_test.go idiom
here since check-go-test-packages requires an external _test package, so
the uploads tests now exercise validation and processing through
ProcessImage/ProcessVideo directly instead of the constructors.