This allows the bazel cache to cache this genrule invocation.
It shouldn't depend on system-specific binaries, so
I believe this is correct.
This is the only part of our build where we otherwise
need to recompute parts after pulling in a full cache.
This fetches the resource directory directly from the released
toolchains, allowing us to stop prebuilding and assembling them.
Moreover insertion of our resource directory is moved to the lua
tracing configuration (solving a `TODO`) and enhanced. Now all options
that start with the original resource directory (either explicit or
implied) are redirected to our resource directory.
This solves a problem where `-I <original resource dir>/some/path` was
passed to the extractor and did not work.
This works around the 5.9 linux compatibility problem by including the
`PackageDescription` swift modules in the in-dist toolchain. Copying the
toolchain and fixing the `-I` flag was not enough as for some reason
compilation of `PackageDescription.swiftinterface` was causing a crash
in the SIL pass. We work around that by pre-compiling those modules
during the build and including `.swiftmodule` files in the resource
directory.
TODO (apart from testing):
* the libraries included in the macOS toolchain are now fat (they were
intel only before), occupying more space. We should see if we need to
trim them down.
* there might be other swiftinterface files causing problems on linux
lurking around...
* if we go with this, we can simplify and trim down the prebuilding we
do leaving out the resource directory.
