Fix #413: Handle inner classes based on content, not names.

Previously, we assumed that if there is a file `Foo` and a file
`Foo$Bar`, that the latter must be an inner class (or companion)
of `Foo`, and that it must therefore be ignored.

As #413 shows, and as the new `JavaDefined$Evil` test shows, this
is not always valid.

The only true test of a class being top-level can only be found in
its content, either through the `Scala` attribute or the
`InnerClasses` attribute.

We completely rewrite `Loaders`. We now systematically try to load
every file that is requested, except when we *already* know that
it is a Java inner class. This means that we will read the contents
of more files than before, but still each file at most once in the
common case.

A file *may* be read multiple times if it is a Java non-top-level
class that gets requested by its name as if it were a top-level
class. The loading will fail but we have to keep the file in case
it is loaded later on as an inner class.

When reading *all* the declarations of a package, we always try
outer classes before potential inner classes so that no double
reading happens.

tasty-query/shared/src/main/scala/tastyquery/Symbols.scala

@@ -1832,6 +1832,9 @@ object Symbols {
     /** Is this the root package? */
     final def isRootPackage: Boolean = owner == null
 
+    /** Is this the empty package? */
+    private[tastyquery] def isEmptyPackage: Boolean = specialKind == SpecialKind.empty
+
     /** Is this the scala package? */
     private[tastyquery] def isScalaPackage: Boolean = specialKind == SpecialKind.scala
 
@@ -1961,9 +1964,10 @@ object Symbols {
     private[Symbols] object SpecialKind:
       inline val None = 0
       inline val root = 1
-      inline val scala = 2
-      inline val java = 3
-      inline val javaLang = 4
+      inline val empty = 2
+      inline val scala = 3
+      inline val java = 4
+      inline val javaLang = 5
     end SpecialKind
 
     private def computeSpecialKind(name: SimpleName, owner: PackageSymbol | Null): SpecialKind =
@@ -1972,6 +1976,7 @@ object Symbols {
           (owner.specialKind: @switch) match
             case SpecialKind.root =>
               name match
+                case nme.EmptyPackageName => SpecialKind.empty
                 case nme.scalaPackageName => SpecialKind.scala
                 case nme.javaPackageName  => SpecialKind.java
                 case _                    => SpecialKind.None