Use simple index for feature/extension/identity

Modules expose these three contructs in their user-facing namespaces.

While the same is available through inference's view of things, and
it is tempting to reuse those, this is a DTO contract over provided
substatements, hence use a simple filter to construct our own view
of the contents.

This reduces the dependency on NamespaceStmtCtx, making it easier
to separate the logic the DTO logic and the inference logic.

Change-Id: I544dd0d2b7f3cf07ea71f1075571056a0c62682d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>