refactor(bazel): extract repository rule helpers

bazel/utils.bzl

@@ -0,0 +1,12 @@
+def require(repository_ctx, executable):
+    path = repository_ctx.which(executable)
+    if path == None:
+        fail("{} must be available in PATH.".format(executable))
+    return path
+
+
+def execute(repository_ctx, arguments):
+    result = repository_ctx.execute(arguments)
+    if result.return_code != 0:
+        fail("command failed: {}\n{}".format(" ".join(arguments), result.stderr))
+    return result.stdout.strip()