Simplify signature context management in Crossgen2 (#32158)

Simplify signature context management in Crossgen2 (#32158)

Based on my recent design discussions with JanK it turns out that
we don't want heterogeneous import cells in composite R2R images.
An interesting corollary of this fact is that the signature context
only changes internally during recursive descent into generic
signatures and at the top level it's a singleton during the entire
build. Due to this fact we don't need to store it in the various
import nodes and keys and we can just use the singleton in
NodeFactory. I have highlighted this fact by renaming it from
InputModuleContext to SignatureContext.

Thanks

Tomas