We don't need to write and pass two separate files to the depscanner,
I've used the pickle because the pickle serializer/deserializer should
be faster than JSON, thought I haven't tested.

We already have to decide whether to scan a file at configure time, so
we don't want to have to do it again at compile time, every time the
depscan rule is run. We can do this by saving and passing the language
to use in the pickle, so depscan doesn't have to re-calculate it. As an
added bonus, this removes an import from depscan

This basically existed for an assert which we don't need, as mypy would
catch that issue for us anyway. Removing the function entirely has some
small performance advantages

Which prevents spurious rebuilds of dyndeps

There's a known ninja bug
(ninja-build/ninja#1952) that running this
with dyndeps will result in Ninja deleting implicit outputs from the
dyndeps, leading to pointless rebuilds. For reference, this is what
CMake does as well.