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>

Library/Homebrew/formula.rb

@@ -377,20 +377,22 @@ def self.factory name
     else
       name = Formula.canonical_name(name)
       # If name was a path or mapped to a cached formula
-      if name.include? "/"
-        require name
 
+      if name.include? "/"
         # require allows filenames to drop the .rb extension, but everything else
         # in our codebase will require an exact and fullpath.
         name = "#{name}.rb" unless name =~ /\.rb$/
 
         path = Pathname.new(name)
         name = path.stem
+
+        require path unless Object.const_defined? self.class_s(name)
+
         install_type = :from_path
         target_file = path.to_s
       else
         # For names, map to the path and then require
-        require Formula.path(name)
+        require Formula.path(name) unless Object.const_defined? self.class_s(name)
         install_type = :from_name
       end
     end