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 <[email protected]>