Simplify output directory handling

[dhellmann]

Fromager uses several flags to control the output directories. We thought we might save some for reuse across runs with different arguments, but that is proving to not be needed. We should simplify the UI by using a single flag for the output directories, and letting the context name subdirectories for different purposes.

[LalatenduMohanty]

@tiran Please add your idea here how we can make it a non breaking change.