New command: zig pkg hash

[DraagrenKirneh]

This PR implements a new package command zig pkg hash which output the package hash the current directory.
Although not directly described as a command to be implemented in #14288 I found it useful to have this command when implementing other package manager stuff, and can see a good use case for including this for user pre/post validating of uploads/downloads of packages later on.

The command will check if either build.zig or build.zig.zon is present or the override flag "--allow-directory" before starting to hash to limit accidental trying to hash a large directory structure by using the command at the wrong place.

It uses a crude directory exclusions of the most common directories not part of a package as #14311 is not implemented yet.

[andrewrk]

Closing:

• over a month old
• not passing the CI
• looks like it's blocking on add inclusion/exclusion directives to build.zig.zon for deciding which files belong the package #14311

I suggest to revisit this once that prerequisite issue is solved.

We should definitely not duplicate the hash logic. That's just asking for trouble.