cmake: Do not open directories as scripts (#14966)

Check if a file path is a directory before opening it.  Extend the
RunCMake.CommandLine test with a case running "cmake -P" on a directory.

Source/cmListFileCache.cxx

@@ -142,7 +142,8 @@ bool cmListFile::ParseFile(const char* filename,
                            bool topLevel,
                            cmMakefile *mf)
 {
-  if(!cmSystemTools::FileExists(filename))
+  if(!cmSystemTools::FileExists(filename) ||
+     cmSystemTools::FileIsDirectory(filename))
     {
     return false;
     }

Tests/RunCMake/CommandLine/P_directory-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CommandLine/P_directory-stderr.txt

@@ -0,0 +1 @@
+^CMake Error: Error processing file: .*/Tests/RunCMake/CommandLine$

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -35,3 +35,5 @@ run_cmake_command(E_sleep-no-args ${CMAKE_COMMAND} -E sleep)
 run_cmake_command(E_sleep-bad-arg1 ${CMAKE_COMMAND} -E sleep x)
 run_cmake_command(E_sleep-bad-arg2 ${CMAKE_COMMAND} -E sleep 1 -1)
 run_cmake_command(E_sleep-one-tenth ${CMAKE_COMMAND} -E sleep 0.1)
+
+run_cmake_command(P_directory ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR})