Flymake: more ambitious cleanup in flymake-mode (bug#69809)

Flymake: more ambitious cleanup in flymake-mode (bug#69809)

Further improve flymake-mode idempotency by not nuke existing overlays.
This means multiple flymake-mode invocations do the same as just one
one, with minimal or no additional side effects.  This is good for
people with lots of 'flymake-mode' in hooks.

The foreign diagnostic importation has been refactored into a separate
function and moved to the "really start" section of 'flymake-start'.
The duplication problem appears to be avoided by some heuristics in
flymake-highlight-line.

A new test has been added.

* lisp/progmodes/flymake.el (flymake--import-foreign-diagnostics): New helper
  (flymake-start): Use it.
  (flymake-mode): Don't nuke overlays here.

* test/lisp/progmodes/flymake-tests.el (foreign-diagnostics): New
  test.

(cherry picked from commit 0ff82eb48725e15bb87a75d4f937b75c2482c59b)