input: tighten layer name rules

Internally, the empty string is used for the main recipes.  Allowing
path separators in the layer name will probably also blow up. Rule out
both.

Some of the type checks were also missing. Make sure that the layer name
is a string and that a layers entry is a dict if not already a string.

pym/bob/input.py

@@ -1739,13 +1739,28 @@ def getScm(self):
         return self.__scm
 
 class LayerValidator:
+    @staticmethod
+    def __validateName(name):
+        if name == "":
+            raise schema.SchemaError("Layer name must not be empty")
+        if any((c in name) for c in '\\/'):
+            raise schema.SchemaError("Invalid character in layer name")
+
     def validate(self, data):
-        if isinstance(data,str):
+        if isinstance(data, str):
+            self.__validateName(data)
             return LayerSpec(data)
+        elif not isinstance(data, dict):
+            raise schema.SchemaUnexpectedTypeError("Layer entry must be a string or a dict", None)
+
         if 'name' not in data:
             raise schema.SchemaMissingKeyError("Missing 'name' key in {}".format(data), None)
+        elif not isinstance(data['name'], str):
+            raise schema.SchemaUnexpectedTypeError("Layer name must be a string", None)
+
         _data = data.copy();
         name = _data.get('name')
+        self.__validateName(name)
         del _data['name']
 
         return LayerSpec(name, RecipeSet.LAYERS_SCM_SCHEMA.validate(_data)[0])