Update check_deps, make reusing scripts easier (#1187)

I went down this route because I figured the installer binary should be in `check_deps`. The result:

- Add thin wrapper scripts for `buildozer` and `bazel`, similar to the `buildifier` wrapper that's already there.
  - These are mainly to avoid deduplicating logic, and dodging intricacies of Python imports that I don't understand well through instead subprocessing.
- Fix update_roots.py (it was missing a `.parent`, must've been relocated without a re-run at some point)
- Fix fuzzer proto code to be `testonly`
- Update the `check_deps` targets

executable_semantics/fuzzing/BUILD

@@ -6,6 +6,7 @@ load("//bazel/fuzzing:rules.bzl", "cc_fuzz_test")
 
 cc_library(
     name = "ast_to_proto_lib",
+    testonly = 1,
     srcs = ["ast_to_proto.cpp"],
     hdrs = ["ast_to_proto.h"],
     deps = [
@@ -19,6 +20,7 @@ cc_library(
 
 cc_library(
     name = "fuzzer_util",
+    testonly = 1,
     srcs = ["fuzzer_util.cpp"],
     hdrs = ["fuzzer_util.h"],
     deps = [
@@ -52,6 +54,7 @@ cc_test(
 
 cc_binary(
     name = "fuzzverter",
+    testonly = 1,
     srcs = ["fuzzverter.cpp"],
     deps = [
         ":ast_to_proto_lib",