Don't initialize symbols before finding classfile

Previously, we would complete symbols before trying to find the
associated classfile. This is problematic for symbols where we don't
have the classfile (for instance, when compiling with Pants'
`strict_deps` enabled).

This initialization was introduced in #758, to workaround a scalac bug
where the associated classfile of a symbol wouldn't be set before the
symbol is completed. This bug has been fixed in Scala 2.12.12 and later
(see scala/scalac#8889).

Fixes #949

internal/compiler-bridge/src/main/scala/xsbt/Dependency.scala

@@ -175,7 +175,7 @@ final class Dependency(val global: CallbackGlobal) extends LocateClassFile with
               isSyntheticCoreClass(targetSymbol)
           )
           if (!noByteCode) {
-            classFile(targetSymbol.initialize) match {
+            classFile(targetSymbol) match {
               case Some((at, binaryClassName)) =>
                 // Associated file is set, so we know which classpath entry it came from
                 processExternalDependency(binaryClassName, at)