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).

src/nvim/CMakeLists.txt

@@ -1,5 +1,7 @@
 option(USE_GCOV "Enable gcov support" OFF)
 
+include(DefCmdTarget)
+
 if(USE_GCOV)
   if(CLANG_TSAN)
     # GCOV and TSAN results in false data race reports
@@ -802,12 +804,22 @@ foreach(sfile ${LINT_NVIM_SOURCES})
 endforeach()
 add_custom_target(lintc DEPENDS ${LINT_TARGETS})
 
+def_cmd_target(lintuncrustify ${UNCRUSTIFY_PRG} UNCRUSTIFY_PRG false)  # Non-fatal so that "lintc" target can depend on it.
+if(UNCRUSTIFY_PRG)
+  add_custom_command(OUTPUT lintuncrustify-cmd APPEND
+    COMMAND ${CMAKE_COMMAND}
+      -DUNCRUSTIFY_PRG=${UNCRUSTIFY_PRG}
+      -DPROJECT_SOURCE_DIR=${PROJECT_SOURCE_DIR}
+      -DLINT_NVIM_SOURCES=${LINT_NVIM_SOURCES}
+      -P ${PROJECT_SOURCE_DIR}/cmake/RunUncrustify.cmake)
+endif()
+
 add_custom_target(
   lintcfull
   COMMAND
     ${LINT_PRG} --suppress-errors=${LINT_SUPPRESS_FILE} ${LINT_NVIM_REL_SOURCES}
   WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
-  DEPENDS ${LINT_PRG} ${LINT_NVIM_SOURCES} ${LINT_SUPPRESS_FILE}
+  DEPENDS ${LINT_PRG} ${LINT_NVIM_SOURCES} ${LINT_SUPPRESS_FILE} lintuncrustify
 )
 
 add_custom_target(generated-sources DEPENDS