rabbitmq-components.mk: Improve top-level DEPS_DIR detection

In addition to the `rabbitmq-components.mk` existence check, we now
verfy that the directory is named `deps`.

This is to increase the chance that, if we find a
`rabbitmq-componentS.mk` file in the upper directories, this project is
indeed inside a DEPS_DIR.

For instance, in our GitHub Actions workflows, when we prepared the
secondary umbrellas for mixed-version testing, it happened that the
secondary umbrellas were under a clone of rabbitmq-server. Therefore
the first (and only) condition was met and the Makefile erroneously
considered it was inside a DEPS_DIR. As a consequence, dependencies of
the umbrellas were fetched in the wrong place.

(cherry picked from commit 9740d03)

mk/rabbitmq-components.mk

@@ -321,15 +321,22 @@ prepare-dist::
 # disable `make distclean` so $(DEPS_DIR) is not accidentally removed.
 
 ifneq ($(wildcard ../../rabbitmq-components.mk),)
-DISABLE_DISTCLEAN = 1
-DEPS_DIR ?= $(abspath ..)
+supposed_deps_dir = $(abspath ..)
 else ifneq ($(wildcard ../../../../rabbitmq-components.mk),)
-DISABLE_DISTCLEAN = 1
-DEPS_DIR ?= $(abspath ../../..)
+supposed_deps_dir = $(abspath ../../..)
 else ifneq ($(wildcard UMBRELLA.md),)
 DISABLE_DISTCLEAN = 1
 endif
 
+# We also verify that the guessed DEPS_DIR is actually named `deps`, to rule
+# out any situation where it is a coincidence that we found a
+# `rabbitmq-components.mk` up upper directories.
+
+ifeq ($(notdir $(supposed_deps_dir)),deps)
+DISABLE_DISTCLEAN = 1
+DEPS_DIR ?= $(supposed_deps_dir)
+endif
+
 ifeq ($(DISABLE_DISTCLEAN),1)
 ifneq ($(filter distclean distclean-deps,$(MAKECMDGOALS)),)
 SKIP_DEPS = 1