Simplify `brew cask doctor`.

Refactor Cask `Artifact`s.

Remove XIP support.

Print warning when reinstalling with invalid options

Delete pkgutil directories that are really files.

Remove `TestCask` class.

This is the same that happens in the command 'install'. By passing
the invalid option names, we enable the formula installer to output
warnings if necessary.

formulary: handle ScriptError in formula

Use array of constants to set up test directories.

Proposed fix for "Failed to eject" errors with DMGs

Allow overriding `MacOS.version`.

I added a new `FormulaUnreadableError` subclass of
`FormulaUnavailableError` so existing `rescue`s of
`FormulaUnavailableError` handle this as well.

The new subclass will output the name of the formula with the error
(because this isn't always obvious from the original exception message)
followed by the original error message.

Fixes #1927.

cask: staged: no sudo in set_permissions

Try "diskutil eject" twice before "diskutil unmount force".

update: note migrations may be performed.

cleanup: consider version_scheme

`brew cleanup` should cleanup formulae which have the older
version scheme.

Don’t use `assert_nil` in `tab_test`.