External Swift Package Manager consumers can only see products declared
in the top-level `Package.swift`. The `VminitdCore` library was declared
inside the `vminitd` subpackage, so it was impossible to build a custom
`vminitd` against it via a URL dependency on `containerization`. This
change moves the `VminitdCore` to the top-level `Package.swift`.
Related to https://github.com/apple/containerization/issues/606
To make sure the project (and many linux specific libraries) builds on
linux lets add a linux step to build for both musl and glibc. Eventually
this will run unit tests as well. This change needs a bit more libc
agnostic-ifying work to make sure things work for both libcs.
We should namespace these a bit, and we need pidfd for some upcoming
terminal work. This also just gets rid of syscall2 and replaces with a
pivot_root wrapper.
Dmitry Kovba