osbuild-mpp: Allow using formating in depsolver node

This is very useful so that mpp-join can create a merged
package list.

tools/osbuild-mpp

@@ -801,6 +801,8 @@ class ManifestFileV1(ManifestFile):
         if not mpp:
             return
 
+        self._process_format(mpp)
+
         packages = element_enter(options, "packages", [])
 
         deps = self.depsolve(solver, mpp)
@@ -884,6 +886,8 @@ class ManifestFileV2(ManifestFile):
         if not mpp:
             return
 
+        self._process_format(mpp)
+
         refs = element_enter(packages, "references", {})
 
         deps = self.depsolve(solver, mpp)