python: better names

- "Normal" instead of "NonSpecial"
- "NonLibrary" instead of "2"

I could not find a good replacement for "NonLibrary", nor for "Source",
but I added QLDocs in a few places to help the reading.

python/ql/test/experimental/dataflow/TestUtil/UnresolvedCalls.qll

@@ -13,7 +13,7 @@ class UnresolvedCallExpectations extends InlineExpectationsTest {
     exists(location.getFile().getRelativePath()) and
     exists(CallNode call |
       not exists(DataFlowPrivate::DataFlowCall dfc | dfc.getNode() = call |
-        // For every `CallNode`, there is a `DataFlowCall` in the form of a `NonSpecialCall`.
+        // For every `CallNode`, there is a `DataFlowCall` in the form of a `NormalCall`.
         // It does not really count, as it has some abstract overrides. For instance, it does not
         // define `getCallable`, so checking for the existence of this guarantees that we are in a
         // properly resolved call.

python/ql/test/experimental/dataflow/calls/test.py

@@ -31,7 +31,7 @@ try:
     # `mypkg.foo` is a `missing module variable`, but `mypkg.subpkg.bar` is compeltely
     # ignored.
     import mypkg
-    mypkg.foo(42) # $ call=mypkg.foo(..) qlclass=NonSpecialCall
-    mypkg.subpkg.bar(43) # $ call=mypkg.subpkg.bar(..) qlclass=NonSpecialCall
+    mypkg.foo(42) # $ call=mypkg.foo(..) qlclass=NormalCall
+    mypkg.subpkg.bar(43) # $ call=mypkg.subpkg.bar(..) qlclass=NormalCall
 except:
     pass