pkgbase: Move headers and libs out of runtime and utilities

Headers from src/include were in the runtime-dev package but
subdirectories of src/include ended up in utilities-dev by default.
Neither package is a good choice - the headers in src/include are not
useful without the libraries contained in clibs-dev.

This moves the standard C headers to clibs-dev (C++ headers are already
in this package). While working on this, I found that various clang
libraries and headers were also bundled into utilities-dev by default
so these are also moved to clang-dev.

I also added a FreeBSD-build-essential meta package to make it simple to
install all the toolchain parts.

PR: 254173
Reviewed byb: manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41815