[NFC] Only Register Primaries As Dependency Sources

Now that the top-level source file is the only dependency source that
matters, the only case that matters is when request evaluation enters
a primary file. For non-primaries, there will be no corresponding
swiftdeps file to emit references into, so we're just wasting time and
memory keeping track of anything that happens there.

This is only possible after we removed cascading dependencies because
unqualified lookups had to be charged to the files they originated in.
Now, we charge those lookups to the primary that initiated the request.

Author: CodaFi

include/swift/AST/EvaluatorDependencies.h

@@ -33,7 +33,7 @@ namespace detail {
 template <typename...> using void_t = void;
 } // namespace detail
 
-// A \c DependencySource is currently defined to be a parent source file.
+// A \c DependencySource is currently defined to be a primary source file.
 //
 // The \c SourceFile instance is an artifact of the current dependency system,
 // and should be scrapped if possible. It currently encodes the idea that
@@ -281,7 +281,7 @@ struct DependencyRecorder {
       auto Source = Req.readDependencySource(coll);
       // If there is no source to introduce, bail. This can occur if
       // a request originates in the context of a module.
-      if (Source.isNull()) {
+      if (Source.isNull() || !Source.get()->isPrimary()) {
         return;
       }
       coll.dependencySources.emplace_back(Source);