The `flask.request` global object is commonly used in request handlers
to access data in the active request. In our modelling, we handled this
by treating the initial (module-local) definition of `request` as a
source of remote flow. In practice this meant a lot of alerts would act
as if `from flask import request` was the ultimate "source" of remote
flow, and to find the actual request-handler-local instance of `request`
one would have to inspect the data-flow path between source and sink.

To improve this state of affairs, I have made the following changes to
the definition of `FlaskRequestSource`:

- We no longer consider `from flask import request` to be a source.
- Instead, we look at all places where that `request` value can flow,
and include only the ones that are `LocalSourceNode`s (so that inside a
request handler, the first occurrence of the `request` object is the
source).

In practice, this leads to alerts that are much easier to decipher.

As it turns out, referring to the request object using `flask.request`
is not uncommon, and this meant restricting to `Name` nodes was too
strong. With the changes in this commit, we now include those
occurrences as well.