Determine a core and core-dev group as foundational packages

[jhuntwork]

These two groups will be essential packages for building, installing and running a minimal Mere system. There is already the build-essential meta package, but I think core-dev might include some other non-essential ones like autotools, possibly perl and python.

This will drive finishing up what should mark the first pass of reviewing the current stable repo.

[jhuntwork]

After reviewing this, I think 'core' should be the minimum required for a minimum mere installation to be reasonably functional. This list will probably grow, but for the moment that is:

• base-layout
• busybox
• mksh
• pacman
• s6

Some of these might bring in dependencies. For example, s6 depends on execline. However, on its own execline isn't considered core. And as mentioned, for greater functionality, other packages may be added in over time, like e2fsprogs or curl and ca-certs. Much of those will depend on how much functionality is desired for a basic system. Also very soon, to finish up #164, mdevd will either be added to the core group or made a dependency of s6. Probably the latter.

core-dev should be able to just replace build-essential:

• linux-headers
• llvm
• m4
• make
• musl-dev
• patch
• pkgconf

I'm not including pacman-build in the above, because while that's needed for building pacman packages, it isn't needed for compiling generic C/C++ code from source.

Everything else in the repo for now should be present because of a dependency in building the above or because the package was useful in initial tests on bare metal. This will get us closer to closing this out and cleaning up the stable repo by promoting testing.