backend/ninja: use a two step process for dependency scanning

This splits the scanner into two discrete steps, one that scans the
source files, and one that that reads in the dependency information and
produces a dyndep.

The scanner uses the JSON format from
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html,
which is the same format the MSVC and Clang use for C++ modules
scanning. This will allow us to more easily move to using MSVC and
clang-scan-deps when possible.

As an added bonus, this correctly tracks dependencies across TU and
Target boundaries, unlike the previous implementation, which assumed
that if it couldn't find a provider that everything was good, but could
run into issues. Because of that limitation Fortran code had to fully
depend on all of it's dependencies, transitive or not. Now, when using
the dep scanner, we can remove that restriction, allowing more
parallelism.

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