Implement dynamic dependencies and remove extra inputs hack

[elpdt852]

Fix #19 🎉

Allows of arbitrarily deep dynamic dependencies, leveraging all the features of dynamic derivations like dynamicOutputs and nix-computed-output placeholders. This lets us remove the "extra-inputs" hack for compiling CppNix where they had bison parser.y generate .cc source files that also need header dependency discovery.

We introduce a "dynamic-task" derivation that detects that when (1) it has "deps = gcc", i.e. it has header dependency discovery, and (2) it has an input that is built rather than opaque, i.e. it is an output from another derivation, then we must have an intermediary derivation that depends on these built inputs and does header discovery and modifies the task derivation to include these discovered inputs.

There's still some work to do, for example we should do discovery in a task per built input rather than together so we can cache discovery more granularly. CppNix has generated inputs that are often shared across build targets, so there is a bit of wasted discovery atm.

With these changes, it makes it possible to build CppNix end to end without any hacks.

ninja-build-src-nix-nix.p-store-info.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
ninja-build-src-nix-nix.p-store.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
ninja-build-src-nix-nix.p-why-depends.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
ninja-build-src-nix-nix.p-upgrade-nix.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
ninja-build-src-nix-nix.p-verify.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
ninja-build-src-nix-nix.p-unix_daemon.cc.o> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths
example-nix> nix-ninja-task: Setup source directory in /build/source/build
example-nix> nix-ninja-task: Linking target src/nix/nix
example-nix> nix-ninja-task: Running: /bin/sh -c "g++  -o src/nix/nix src/nix/nix.p/add-to-store.cc.o src/nix/nix.p/app.cc.o src/nix/nix.p/self-exe.cc.o src/nix/nix.p/build.cc.o src/nix/nix.p/bundle.cc.o src/nix/nix.p/cat.cc.o src/nix/nix.p/config-check.cc.o src/nix/nix.p/config.cc.o src/nix/nix.p/copy.cc.o src/nix/nix.p/crash-handler.cc.o src/nix/nix.p/derivation-add.cc.o src/nix/nix.p/derivation-show.cc.o src/nix/nix.p/derivation.cc.o src/nix/nix.p/develop.cc.o src/nix/nix.p/diff-closures.cc.o src/nix/nix.p/dump-path.cc.o src/nix/nix.p/edit.cc.o src/nix/nix.p/env.cc.o src/nix/nix.p/eval.cc.o src/nix/nix.p/flake.cc.o src/nix/nix.p/fmt.cc.o src/nix/nix.p/hash.cc.o src/nix/nix.p/log.cc.o src/nix/nix.p/ls.cc.o src/nix/nix.p/main.cc.o src/nix/nix.p/make-content-addressed.cc.o src/nix/nix.p/man-pages.cc.o src/nix/nix.p/nar.cc.o src/nix/nix.p/optimise-store.cc.o src/nix/nix.p/path-from-hash-part.cc.o src/nix/nix.p/path-info.cc.o src/nix/nix.p/prefetch.cc.o src/nix/nix.p/profile.cc.o src/nix/nix.p/realisation.cc.o src/nix/nix.p/registry.cc.o src/nix/nix.p/repl.cc.o src/nix/nix.p/run.cc.o src/nix/nix.p/search.cc.o src/nix/nix.p/sigs.cc.o src/nix/nix.p/store-copy-log.cc.o src/nix/nix.p/store-delete.cc.o src/nix/nix.p/store-gc.cc.o src/nix/nix.p/store-info.cc.o src/nix/nix.p/store-repair.cc.o src/nix/nix.p/store.cc.o src/nix/nix.p/upgrade-nix.cc.o src/nix/nix.p/verify.cc.o src/nix/nix.p/why-depends.cc.o src/nix/nix.p/unix_daemon.cc.o src/nix/nix.p/nix-build_nix-build.cc.o src/nix/nix.p/nix-collect-garbage_nix-collect-garbage.cc.o src/nix/nix.p/nix-copy-closure_nix-copy-closure.cc.o src/nix/nix.p/nix-env_nix-env.cc.o src/nix/nix.p/nix-env_user-env.cc.o src/nix/nix.p/nix-instantiate_nix-instantiate.cc.o src/nix/nix.p/nix-store_dotgraph.cc.o src/nix/nix.p/nix-store_graphml.cc.o src/nix/nix.p/nix-store_nix-store.cc.o src/nix/nix.p/build-remote_build-remote.cc.o src/nix/nix.p/nix-channel_nix-channel.cc.o -Wl,--as-needed -Wl,--no-undefined '-Wl,-rpath,$ORIGIN/../libutil:$ORIGIN/../libstore:$ORIGIN/../libexpr:$ORIGIN/../libfetchers:$ORIGIN/../libflake:$ORIGIN/../libmain:$ORIGIN/../libcmd' -Wl,--start-group src/libutil/libnixutil.so src/libstore/libnixstore.so src/libexpr/libnixexpr.so src/libfetchers/libnixfetchers.so src/libflake/libnixflake.so src/libmain/libnixmain.so src/libcmd/libnixcmd.so /nix/store/9fz37cc2hai6n2d17fdr3gcdr67x3853-libarchive-3.8.1-lib/lib/libarchive.so /nix/store/3dzksnrd6a17b7qlrhrvxmbs4mlc2n9c-boehm-gc-8.2.8/lib/libgc.so -lpthread -ldl -Wl,--end-group -pthread"
example-nix> nix-ninja-task: Fixed RPATH for src/nix/nix
example-nix> nix-ninja-task: Finished! Copying 1 build outputs to derivation output paths

$ ./result-src-nix-nix/src/nix/nix --version
nix (Nix) 2.27.1