Ninja: NFC: refactor swift module name computations

In order to handle determining the swiftmodule name to add to the ninja
dependency graph, we'll need to be able to compute the swiftmodule name
for the dependency target, not just the current target. This patch
refactors the computation of the module name out of inaccessible lambdas
and into static functions that can compute the swiftmodule name from the
generator and the target.