Make builds faster by caching ALL_GO_DIRS

This operation takes 2-5 seconds on every build, but doesn't actually need to
run most of the time.  Now we cache it and see if it needs a rebuild (fast)
before actually rebuilding (slow).

hack/make-rules/helpers/cache_go_dirs.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+
+# Copyright 2014 The Kubernetes Authors.
+#
+# 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.
+
+# This script finds, caches, and prints a list of all directories that hold
+# *.go files.  If any directory is newer than the cache, re-find everything and
+# update the cache.  Otherwise use the cached file.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+if [[ -z "${1:-}" ]]; then
+    echo "usage: $0 <cache-file>"
+    exit 1
+fi
+CACHE="$1"; shift
+
+# This is a partial 'find' command.  The caller is expected to pass the
+# remaining arguments.
+#
+# Example:
+#   kfind -type f -name foobar.go
+function kfind() {
+    find .                         \
+        -not \(                    \
+            \(                     \
+                -path ./vendor -o  \
+                -path ./_\* -o     \
+                -path ./.\* -o     \
+                -path ./docs -o    \
+                -path ./examples   \
+            \) -prune              \
+        \)                         \
+        "$@"
+}
+
+NEED_FIND=true
+# It's *significantly* faster to check whether any directories are newer than
+# the cache than to blindly rebuild it.
+if [[ -f "${CACHE}" ]]; then
+    N=$(kfind -type d -newer "${CACHE}" -print -quit | wc -l)
+    [[ "${N}" == 0 ]] && NEED_FIND=false
+fi
+mkdir -p $(dirname "${CACHE}")
+if $("${NEED_FIND}"); then
+    kfind -type f -name \*.go  \
+        | xargs -n1 dirname    \
+        | sort -u              \
+        | sed 's|^./||'        \
+        > "${CACHE}"
+fi
+cat "${CACHE}"