Remove global methods from formulae

Removes any global methods from formulae, and moves #kext_prefix (which
seems to be at least somewhat abstractable) into the Formula class. The
only formula with global methods is now aspell; it (and its generating
script in contrib) has been changed to prefix that method with
`aspell_`, to minimize the risk of name collisions.

Closes #19331.
Closes #19343.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>