Fix undigested m.jar under validated targets (pantsbuild#8479)

### Problem

For hermetic rsc+zinc, m.jar in validated targets are not digested, causing downstream targets to error out with missing digest. E.g.
```
zinc[rsc-and-zinc](<target>) failed: ClasspathEntry ClasspathEntry(path='/Users/user/workspace/s2/.pants.d/compile/rsc/c1e3836b60e5/<target_id>/current/rsc/m.jar', directory_digest=None) didn't have a Digest, so won't be present for hermetic execution of zinc
                   Traceback:
                     File "/Users/user/workspace/pants/src/python/pants/backend/jvm/tasks/jvm_compile/execution_graph.py", line 281, in worker
                       work()
                   
                     File "/Users/user/workspace/pants/src/python/pants/backend/jvm/tasks/jvm_compile/execution_graph.py", line 42, in __call__
                       self.fn()
                   
                     File "/Users/user/workspace/pants/src/python/pants/backend/jvm/tasks/jvm_compile/jvm_compile.py", line 982, in _default_work_for_vts
                       counter)
                   
                     File "/Users/user/workspace/pants/src/python/pants/backend/jvm/tasks/jvm_compile/jvm_compile.py", line 593, in _compile_vts
                       self._get_plugin_map('scalac', ScalaPlatform.global_instance(), ctx.target),
```

### Solution

Make sure m.jar in validated targets are digested.

src/python/pants/backend/jvm/tasks/jvm_compile/rsc/rsc_compile.py

@@ -232,8 +232,20 @@ def confify(entries):
     # Ensure that the jar/rsc jar is on the rsc_mixed_compile_classpath.
     for target in targets:
       merged_cc = compile_contexts[target]
-      rsc_cc = merged_cc.rsc_cc
       zinc_cc = merged_cc.zinc_cc
+      rsc_cc = merged_cc.rsc_cc
+      # Make sure m.jar is digested if it exists when the target is validated.
+      if rsc_cc.rsc_jar_file.directory_digest is None and os.path.exists(rsc_cc.rsc_jar_file.path):
+        relpath = fast_relpath(rsc_cc.rsc_jar_file.path, get_buildroot())
+        classes_dir_snapshot, = self.context._scheduler.capture_snapshots([
+          PathGlobsAndRoot(
+            PathGlobs([relpath]),
+            get_buildroot(),
+            Digest.load(relpath),
+          ),
+        ])
+        rsc_cc.rsc_jar_file.hydrate_missing_directory_digest(classes_dir_snapshot.directory_digest)
+
       if rsc_cc.workflow is not None:
         cp_entries = rsc_cc.workflow.match({
           self.JvmCompileWorkflowType.zinc_only: lambda: confify([self._classpath_for_context(zinc_cc)]),