Fix calculation of $(1)_DEP_PKGS_SHA in Makefile.cache (#10764)

In Makefile.cache, for $(1)_DEP_PKGS_SHA, the intention is to include
the DEP_MOD_SHA and MOD_HASH of each of the current package's
dependencies. However, there's a level of dereferencing missing; instead
of grabbing the value of $(dfile)_DEP_MOD_SHA, it is literally using the
variable name $(dfile)_DEP_MOD_SHA. This means that the value of this
variable will not change when some dependency changes.

The impact of this is in transitive dependencies. For a specific
example, if there is some change in sairedis, then sairedis will be
rebuilt (because there's a change within that component), and swss will
be rebuilt (because it's a direct dependency), but
docker-swss-layer-buster will not get rebuilt, because only the direct
dependencies are effectively being checked, and those aren't changing.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Makefile.cache

@@ -186,7 +186,7 @@ define GET_MOD_DEP_SHA
 	$(if $($(1)_DEP_FILES_MISSING), $(warning "[ DPKG ] Dependecy file(s) are not found for $(1) : $($(1)_DEP_FILES_MISSING)))
 
 	# Include package dependencies hash values into package hash calculation
-	$(eval $(1)_DEP_PKGS_SHA := $(foreach dfile,$(1)_MOD_DEP_PKGS,$(dfile)_DEP_MOD_SHA $(dfile)_MOD_HASH))
+	$(eval $(1)_DEP_PKGS_SHA := $(foreach dfile,$($(1)_MOD_DEP_PKGS),$($(dfile)_DEP_MOD_SHA) $($(dfile)_MOD_HASH)))
 
 	$(eval $(1)_DEP_MOD_SHA := $(shell bash -c "git hash-object  $($(1)_DEP_MOD_SHA_FILES) && echo $($(1)_DEP_PKGS_SHA)" \
                             | sha1sum | awk '{print substr($$1,0,23);}'))