Added a filegroup for prelude.carbon for easier referencing from other BUILD files and to help with internal build visibility issue (#1160)

* added a filegroup for prelude.carbon

* renamed prelude_files filegroup to standard_libraries per code review comment

executable_semantics/BUILD

@@ -9,10 +9,15 @@ package(default_visibility = [
     "//executable_semantics:__subpackages__",
 ])
 
+filegroup(
+    name = "standard_libraries",
+    srcs = ["data/prelude.carbon"],
+)
+
 cc_binary(
     name = "executable_semantics",
     srcs = ["main.cpp"],
-    data = ["data/prelude.carbon"],
+    data = [":standard_libraries"],
     deps = [
         "//common:error",
         "//executable_semantics/common:arena",

executable_semantics/fuzzing/BUILD

@@ -19,11 +19,11 @@ cc_test(
     name = "ast_to_proto_test",
     srcs = ["ast_to_proto_test.cpp"],
     args = [
-        "$(location //executable_semantics:data/prelude.carbon)",
+        "$(locations //executable_semantics:standard_libraries)",
         "$(locations //executable_semantics/testdata:carbon_files)",
     ],
     data = [
-        "//executable_semantics:data/prelude.carbon",
+        "//executable_semantics:standard_libraries",
         "//executable_semantics/testdata:carbon_files",
     ],
     deps = [