racket/base: add `#:for-module?` argument to `open-input-file`

Exposes a feature that is used by the default load handler to
raise `exn:fail:{syntax,filesystem}:missing-module` exceptions.
racket/gui: adapt `text-editor-load-handler` to missing-module exception

Raise `exn:fail:{filesystem,syntax}:missing-module` instead of
`exn:fail:filesystem` when a module file does not exist.