build(lint): fix luacheck not found #18940

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).

cmake/DefCmdTarget.cmake

@@ -0,0 +1,27 @@
+# Defines a target named ${target} and a command with (symbolic) output
+# ${target}-cmd. If ${prg} is undefined the target prints "not found".
+#
+# - Use add_custom_command(…APPEND) to build the command after this.
+# - Use add_custom_target(…DEPENDS) to run the command from a target.
+function(def_cmd_target target prg prg_name prg_fatal)
+  # Define a mostly-empty command, which can be appended-to.
+  add_custom_command(OUTPUT ${target}-cmd
+      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+      COMMAND ${CMAKE_COMMAND} -E echo "${target}"
+  )
+  # Symbolic (does not generate an artifact).
+  set_source_files_properties(${target}-cmd PROPERTIES SYMBOLIC "true")
+
+  if(prg OR NOT prg_fatal)
+    add_custom_target(${target}
+      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+      DEPENDS ${target}-cmd)
+    if(NOT prg)
+      add_custom_command(OUTPUT ${target}-cmd APPEND
+        COMMAND ${CMAKE_COMMAND} -E echo "${target}: SKIP: ${prg_name} not found")
+    endif()
+  else()
+    add_custom_target(${target} false
+      COMMENT "${target}: ${prg_name} not found")
+  endif()
+endfunction()