What runs from the kept declaration says Data as the module's manifest does, and only a have that runs counts at the start (issue 924)

The kept-declaration fallback built a composition without Data, so after a restart a data module (mongodb, mssql
on novox) was judged to move its data against its first composition and the operator was asked a false yes. Now
every such composition takes Data from the manifest (agent.Env.HoldsData, the composer's DataItems), and a module
whose manifest cannot be read is said to be unjudgeable rather than taken to hold none. The composition a have
names comes before the kept declaration in the judgement too.

loadHad takes a have only in the phases baseline takes as running (running, held, a composer's mark): one backing
up, applying, checking, failed or waiting names a composition not applied (review of 031e106). Two rows added to
the same table test; the count stays.

internal/selfcompose/selfcompose.go

@@ -757,6 +757,27 @@ func (c *Composer) Policy(ctx context.Context, module string) string {
 	return policy
 }
 
+// HoldsData says whether a module's manifest declares data (catalogue.Manifest.DataItems), as a composition's Data says
+// it, and whether the manifest could be read: the agent's Data for what runs of a module from the kept declaration
+// (agent.Env.HoldsData, issue 924).
+func (c *Composer) HoldsData(ctx context.Context, module string) (data, known bool) {
+	c.mu.Lock()
+	m, ok := c.modules[module]
+	c.mu.Unlock()
+	if !ok || m.Manifest == "" {
+		return false, false
+	}
+	raw, err := reader{c: c}.Manifest(ctx, m.Manifest)
+	if err != nil {
+		return false, false
+	}
+	parsed, err := catalogue.ParseManifestForInputs(raw)
+	if err != nil {
+		return false, false
+	}
+	return len(parsed.DataItems()) > 0, true
+}
+
 // reader is the composer's view of the bus as frombus reads it.
 type reader struct{ c *Composer }