factory: don't reload previously defined formulae

build.rb calls Formula.factory to get a usable Formula object to pass to
its install method. However, because the formula file is the actual
executing script, its class is already defined, and loading it again
causes the class to be re-evaluated, which, unfortunately, is not
idempotent.

This bug has existed for a very long time, and its side effects include
duplicate entries the deps array and mirrors array, among others.
Fortunately, the fix is very simple.

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