move antlr generated CEL parser to it's own package

This fixes a build failure on MacOS due to path conflict with
CelParser.java.

Author: mikedanese

parser/BUILD.bazel

@@ -26,7 +26,7 @@ java_library(
 java_library(
     name = "cel_g4_visitors",
     visibility = ["//visibility:public"],
-    exports = ["//parser/src/main/java/dev/cel/parser:cel_g4_visitors"],
+    exports = ["//parser/src/main/java/dev/cel/parser/gen:cel_g4_visitors"],
 )
 
 java_library(

parser/src/main/java/dev/cel/parser/BUILD.bazel

@@ -44,7 +44,6 @@ java_library(
     tags = [
     ],
     deps = [
-        ":cel_g4_visitors",
         ":macro",
         ":operator",
         ":parser_builder",
@@ -54,6 +53,7 @@ java_library(
         "//common/annotations",
         "//common/ast",
         "//common/internal",
+        "//parser:cel_g4_visitors",
         "@maven//:com_google_errorprone_error_prone_annotations",
         "@maven//:com_google_guava_guava",
         "@maven//:org_antlr_antlr4_runtime",
@@ -103,24 +103,6 @@ java_library(
     ],
 )
 
-antlr4_java_combined(
-    name = "cel_g4",
-    src = "CEL.g4",
-    listener = False,
-    package = "cel.parser.internal",
-    visitor = True,
-)
-
-java_library(
-    name = "cel_g4_visitors",
-    srcs = [":cel_g4"],
-    tags = [
-    ],
-    deps = [
-        "@maven//:org_antlr_antlr4_runtime",
-    ],
-)
-
 java_library(
     name = "unparser",
     srcs = UNPARSER_SOURCES,

parser/src/main/java/dev/cel/parser/gen/BUILD.bazel

@@ -0,0 +1,27 @@
+"""
+The generated parser is kept seperate from //parser/src/main/java/dev/cel/parser
+to avoid a path conflict with parser/src/main/java/dev/cel/parser/CelParser.java
+that causes build failures on filesystems with case-insensitive paths (e.g. macOS).
+"""
+load("//:antlr.bzl", "antlr4_java_combined")
+
+antlr4_java_combined(
+    name = "cel_g4",
+    src = "CEL.g4",
+    listener = False,
+    package = "cel.parser.internal",
+    visitor = True,
+)
+
+java_library(
+    name = "cel_g4_visitors",
+    srcs = [":cel_g4"],
+    tags = [
+    ],
+    deps = [
+        "@maven//:org_antlr_antlr4_runtime",
+    ],
+    visibility = [
+        "//parser:__pkg__",
+    ]
+)