dune exec needs to add .exe on Windows

dune exec -- foo on Windows needs to consider dune exec -- foo.exe when working out if there is a program to (re)compile. At the moment, the behaviour is mixed:

• if _build/install/default/bin/foo.exe has previously been built, then you get a potentially stale execution of foo if its sources have changed
• if _build/install/default/bin/foo.exe hasn't previously been built, then you get an error.

Specifying dune exec -- foo.exe works consistently, but this has two problems:

• It's surprising on Windows (unless the user has done something really weird with the PATHEXT environment variable)
• It's not good for a cross-platform set-up since dune exec -- foo.exe will fail on a (Unix) system which installs foo.exe as foo with the executable bit set.

(Dune 2.4.0; Windows mingw32; OCaml 4.08.1)

Start at the dune exec command path and reproduce the reported Windows behavior with foo and foo.exe. Trace how the executable is located and how source changes trigger rebuilding; it is done when dune exec -- foo consistently handles the Windows .exe name without breaking the corresponding Unix setup.