Fix issue where bazel-force-enabled-modules aren't actually analyzed.

Bug: 273910287
Test: run mixed_mode_test.sh

Change-Id: I309c423b6e7f505ad1db0d9327e7de6b8298dfe9
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index fde9b69..b7678a4 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -321,6 +321,9 @@
 					// target, each of a different rule class.
 					metrics.IncrementRuleClassCount(t.ruleClass)
 				}
+			} else if _, ok := ctx.Config().BazelModulesForceEnabledByFlag()[m.Name()]; ok && m.Name() != "" {
+				err := fmt.Errorf("Force Enabled Module %s not converted", m.Name())
+				errs = append(errs, err)
 			} else {
 				metrics.AddUnconvertedModule(moduleType)
 				return