Support `.dmg` and `.pkg` formats

[magistau]

These are common for macOS applications — the former is similar to .iso in that it is usually treated as a disk image, the latter seems to be "xar archive"; crates seem to exist for both, but they're quite unpopular, and I'm not sure which are better.

Crates with .dmg support:

• dmgwiz
• apple-dmg
• dmg
• dmg-oxide

Crates with .pkg support:

• apple-xar
• zar
• apple-flat-package
• xar