Problem:
Since 6d57bb89c1#18543, luacheck is not found on some systems when
running the "lintlua" target.
Solution:
- Move the find_program() to the top-level CMakeLists.txt
- Define a def_cmd_target() function with fewer assumptions than the old
lint() function.
- Move "lintuncrustify" to src/nvim/CMakeLists.txt so it can reuse the
$LINT_NVIM_SOURCES already defined there.
- Make the lint targets _fatal_ by default. There is little reason for
the "lint" umbrella target defined in Makefile to exist if it's going
to ignore the absence of the actual linters.
- For now, keep the uncrustify call in a separate cmake script so that
it can be silenced (too noisy).
