Harden ImportSuggestions

I got an error:
```
error while loading Enum$,
class file /Users/odersky/workspace/dotty/library/target/scala-0.27/classes/scala/Enum.class is broken, reading aborted with class dotty.tools.tasty.UnpickleException
TASTy signature has wrong version.
 expected: 24.0
 found   : 23.0
```
even though there was no enum in my program. It turned out that the error was generated when ImportSuggestions tried to access an outdated
class file on the class path. This shoul dnot cause an error; instead the class file should be just ignored.

Author: odersky

compiler/src/dotty/tools/dotc/typer/ImportSuggestions.scala

@@ -13,6 +13,7 @@ import ast.{untpd, tpd}
 import Implicits.{hasExtMethod, Candidate}
 import java.util.{Timer, TimerTask}
 import collection.mutable
+import scala.util.control.NonFatal
 
 /** This trait defines the method `importSuggestionAddendum` that adds an addendum
  *  to error messages suggesting additional imports.
@@ -58,10 +59,12 @@ trait ImportSuggestions:
     val seen = mutable.Set[TermRef]()
 
     def lookInside(root: Symbol)(using Context): Boolean =
-      if root.is(Package) then root.isTerm && root.isCompleted
-      else !root.name.is(FlatName)
-        && !root.name.lastPart.contains('$')
-        && root.is(ModuleVal, butNot = JavaDefined)
+      explore {
+        if root.is(Package) then root.isTerm && root.isCompleted
+        else !root.name.is(FlatName)
+          && !root.name.lastPart.contains('$')
+          && root.is(ModuleVal, butNot = JavaDefined)
+      }
 
     def nestedRoots(site: Type)(using Context): List[Symbol] =
       val seenNames = mutable.Set[Name]()
@@ -245,12 +248,11 @@ trait ImportSuggestions:
         match
           case (Nil, partials) => (extensionImports, partials)
           case givenImports => givenImports
-    catch
-      case ex: Throwable =>
-        if ctx.settings.Ydebug.value then
-          println("caught exception when searching for suggestions")
-          ex.printStackTrace()
-        (Nil, Nil)
+    catch case NonFatal(ex) =>
+      if ctx.settings.Ydebug.value then
+        println("caught exception when searching for suggestions")
+        ex.printStackTrace()
+      (Nil, Nil)
     finally
       timer.cancel()
       reduceTimeBudget(((System.currentTimeMillis() - start) min Int.MaxValue).toInt)