Add `deps` support to `zig_library` and `zig_binary`

[darthfork]

The current API only accepts source files. A usable language ruleset needs a dependency model.

Current code:

• zig/rules.bzl

Tasks:

• Design provider flow for Zig libraries.
• Add deps attributes to zig_library and zig_binary.
• Ensure transitive source/archive information is propagated correctly.
• Decide how local package/module imports map onto Bazel dependencies.

Acceptance criteria:

• zig_library can depend on another zig_library.
• zig_binary can depend on one or more zig_library targets.
• Example targets demonstrate dependency composition.

Parent issue: #3

[darthfork]

Completed by #23, which added deps support to zig_library and zig_binary, propagated transitive module/archive information, and added a deps example.