This is experimental and will be reverted if it causes problems.

Closes Homebrew/homebrew#30392.

There is a nasty circular dependency here:

formula.rb
  requirement.rb
    extend/ENV.rb
      extend/ENV/shared.rb
        formula.rb

Probably the information that the build environment needs from formula
should be passed in at runtime, but that seems hard and I really just
want to be able to run the tests with warnings turned on. :/

All callers of this method now pass a mode object.

delete/unlink on a directory is the same operation as rmdir.

Fixes Homebrew/homebrew#30558.

This is one of the many problems with using OpenStruct.