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 <[email protected]>
Co-authored-by: Mathias Fredriksson <[email protected]>
(cherry picked from commit e6c8c66)

Author: johnstcn