add stack resolution: search path, references, overlays, groups

Replaces the 500-character stack_path expression of
make/apps/def.docker.mk:159 and its leaked $(stackz) loop variable with
three readable functions, covered by 20 assertions.

spec/unit/stack_version_helper.sh

@@ -0,0 +1,5 @@
+#shellcheck shell=sh
+# Helper: myos_stack_resolve exports the parsed name and version as a side
+# effect, which a subshell would discard.
+myos_stack_resolve "postgres:9.6" >/dev/null
+printf '%s %s\n' "$MYOS_STACK_NAME" "$MYOS_STACK_VERSION"