Say why assign finds no module, and keep an assignment pending on its build

A merge asks for a new module's build, and assign answered "no module of that
name" until the build registered it, which read as a module nobody registered
(hq issue 325). Keep every build request, tell a build in flight, a module
known and not built, and an unknown name apart, and make an assignment made
while the build runs when the build registers the module.

cmd/mesh-controller/release_plan.go

@@ -337,8 +337,15 @@ func askTier(ctx context.Context, inv *inventory.Inventory, p *inventory.Plan) e
 
 // askABuild is how a plan asks for one build, not waited for, and learns the id it asked under. A
 // variable so a test of what a plan does around an ask needs no build machine.
-var askABuild = func(ctx context.Context, source buildSource, path, ref string) (string, error) {
-	return buildOneAsked(ctx, source, path, ref, 0, false)
+//
+// Set in init, not where it is declared: an assignment pending on a build asks for one too (novox/hq issue
+// 325), and a build's outcome makes pending assignments, so the two refer to each other.
+var askABuild func(ctx context.Context, source buildSource, path, ref string) (string, error)
+
+func init() {
+	askABuild = func(ctx context.Context, source buildSource, path, ref string) (string, error) {
+		return buildOneAsked(ctx, source, path, ref, 0, false)
+	}
 }
 
 // askModule asks the build machine for one module of a plan and marks it asked, with the id it was