Add starlark debugger python script interface

This patch will add a python script interface to the debugger together
with the following tools:
* Interactive debugger
* Trace tool

See README.md for more information

src/main/java/com/google/devtools/build/lib/starlarkdebug/proto/BUILD

@@ -1,5 +1,6 @@
 load("@com_google_protobuf//bazel:java_proto_library.bzl", "java_proto_library")
 load("@com_google_protobuf//bazel:proto_library.bzl", "proto_library")
+load("@com_google_protobuf//bazel:py_proto_library.bzl", "py_proto_library")
 load("//tools/build_rules:utilities.bzl", "java_library_srcs")
 
 package(default_visibility = ["//src:__subpackages__"])
@@ -28,3 +29,18 @@ proto_library(
     srcs = ["starlark_debugging.proto"],
     visibility = ["//visibility:public"],
 )
+
+py_proto_library(
+    name = "starlark_debugging_py_proto",
+    deps = [":starlark_debugging_proto"],
+    visibility = ["//visibility:public"],
+)
+
+# Used by tools/starlarkdebug
+py_library(
+    name = "starlark_debugging_py_proto_library",
+    srcs = [":starlark_debugging_py_proto"],
+    deps = ["@com_google_protobuf//:protobuf_python"],
+    imports = ["."], # Avoid very long import path
+    visibility = ["//visibility:public"],
+)

tools/starlarkdebug/BUILD

@@ -0,0 +1,30 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+package(default_visibility = ["//visibility:public"])
+
+py_library(
+    name = "debugger",
+    srcs = ["debugger.py"],
+    deps = [
+        "//src/main/java/com/google/devtools/build/lib/starlarkdebug/proto:starlark_debugging_py_proto_library",
+    ],
+    imports = ["."],
+)
+
+py_binary(
+    name = "trace",
+    srcs = ["trace.py"],
+    deps = [":debugger"],
+)

tools/starlarkdebug/README.md

@@ -0,0 +1,58 @@
+# Starlark debug
+
+This is the debugger and trace tool presented at Build Meetup 2021 (tutorial in video):
+https://www.youtube.com/watch?v=D7-BbD6QjeU
+
+## Preconditions
+
+Since you cannot start python in interactive mode using `bazel run` you need
+to build the starlark_debugger protobuf interface and then either copy it
+alongside the debugger tool or pass it's location in the PYTHONPATH
+environment variable.
+
+```
+STARLARK_DEBUGGER_PROTO=src/main/java/com/google/devtools/build/lib/starlarkdebug/proto
+bazel build //${STARLARK_DEBUGGER_PROTO}:starlark_debugging_py_proto
+export PYTHONPATH="$(realpath bazel-bin/${STARLARK_DEBUGGER_PROTO})"
+python3 tools/starlarkdebug/debugger.py --help
+```
+
+Note that above will use bazel to build the python interface but uses a local
+python installation to run the debugger. This installation needs the protobuf
+package installed which may fail due to mismatching protobuf versions in which
+you can:
+* Create a virtual python environment and install the protobuf version bazel uses there
+* Use protoc tool to generate the debugging interface with your version
+
+## Setup bazel for debugging
+
+Bazel needs to be running with the debugger active for the debugger tool to run
+otherwise you will get a `connection refused` message or similar. 
+
+Execute the command to debug as normal but add a few extra build flags for
+it to wait for the debugger tool to connect. Here is a sample bazelrc file:
+```
+build:starlark_debug # Enable the skylark debugger
+build:starlark_debug --experimental_skylark_debug
+build:starlark_debug --experimental_skylark_debug_server_port=7200
+build:starlark_debug --keep_state_after_build=false
+```
+
+You can optionally add `--build=false` to the command
+line as the build phase is not relevant for the starlark debug.
+
+Note if bazel exits immediately without triggering a breakpoint you may
+need to run `bazel shutdown` to clear the analysis cache. Disabling  the
+`keep_state_after_build` flag (as in the example above) will prevent bazel
+to cache the execution in memory.
+
+## Interactive debugger
+
+The easiest usage as shown in the tutorial is to first execute the command
+to debug as normal with additional debug flags and then execute the debugger
+in interactive mode in another terminal while bazel waits for a debugger to
+attach.
+
+```
+python3 -i tools/starlarkdebug/debugger.py
+```