bug#29687: flymake aliases should be before defcustoms

[Glenn Morris]

Package: emacs
Version: 26.0.90

A bunch of flymake options have been renamed; eg flymake-xml-program.
The define-obsolete-variable-alias statements are at the end of the file.
These statements need to be moved to before the associated defcustoms,
so that existing user customizations are respected (as explained in the
doc of define-obsolete-variable-alias).