Use consistent names for dialect op source files

Use consistent names for dialect op source files

This CL changes dialect op source files (.h, .cpp, .td) to follow the following
convention:

  <full-dialect-name>/<dialect-namespace>Ops.{h|cpp|td}

Builtin and standard dialects are specially treated, though. Both of them do
not have dialect namespace; the former is still named as BuiltinOps.* and the
latter is named as Ops.*.

Purely mechanical. NFC.

PiperOrigin-RevId: 236371358