…ted but still working

- What I did
Added pkg build --push as a better alternative to pkg push, but still keeping pkg push as functioning (but marked deprecated).

It may not seem like much of a difference, but it has two benefits:

First, to users, the flags are more consistent. Flags should always be for one of:

• a command (Flags)
• a command and all of its subcommands (Persistent Flags)

This weird hybrid, where all of the flags were shared for pkg push and pkg build, but not any other pkg subcommands, was confusing. Now, these are pkg build commands.

Second, to us, the code supporting it is cleaner and simpler.

- How I did it
Removed the shared function, made it part of pkgBuild() and changed pkgPush() to just call build after setting --push flag to true.

- How to verify it
CI will check that there are no regressions. I tested each step, including making sure flags get carried over.

- Description for the changelog

pkg build --push replaces pkg push