Python: Fix resolution of relative imports from namespace packages

The fix may look a bit obscure, so here's what's going on.

When we see `from . import helper`, we create an `ImportExpr` with level
equal to 1 (corresponding to the number of dots). To resolve such
imports, we compute the name of the enclosing package, as part of
`ImportExpr.qualifiedTopName()`. For this form of import expression, it
is equivalent to `this.getEnclosingModule().getPackageName()`. But
`qualifiedTopName` requires that `valid_module_name` holds for its
result, and this was _not_ the case for namespace packages.

To fix this, we extend `valid_module_name` to include the module names
of _any_ folder, not just regular package (which are the ones where
there's a `__init__.py` in the folder). Note that this doesn't simply
include all folders -- only the ones that result in valid module names
in Python.

python/ql/test/experimental/import-resolution-namespace-relative/pkg/helper.py

@@ -1,2 +1,2 @@
 def process(value):
-    sink(value) #$ MISSING: prints=source
+    sink(value) #$ prints=source