Problem
-------
`ActiveSupportConcern` uses
[const_defined?](https://apidock.com/ruby/Module/const_defined%3f) to
check if `ClassMethods` exists on a module. By default, `const_defined?`
checks the module and its ancestors for the existence of the constant.
This means that `const_defined?` will return `true` when a module does
not have `ClassMethods` defined, but one of its ancestors does. This can
result in `ActiveSupportConcern` generating
`mixes_in_class_methods.*ClassMethods` lines for invalid
`.*ClassMethods` modules.

The first commit in this PR adds a negative test case that should fail
with the existing code.

Solution
--------
`const_defined?` takes a second parameter that controls whether to
include ancestors in the check. Setting this parameter to `false` to
exclude ancestors.

Set the second parameter of
[const_defined?](https://apidock.com/ruby/Module/const_defined%3f) to
false in order to exclude ancestors of the module when checking for the
existence of the `ClassMethods` constant.

…method