fix: allow setting MagicDir in Options (#337)

- Unexports constants.MagicDir and moves it to an internal package.
- Moves other const/var declarations in `constants` package to where they are used
- Removes constants package
- Adds MagicDirBase to options (intentionally not configurable via CLI)
- Removes usages of default MagicDir to instead use path provided by options

Co-authored-by: Danny Kopping <dannykopping@gmail.com>
Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

Author: 3 people