An attempt to normalize some of the function names in build.zig. Normalize add*Dir to add*Path. Also use "Library" instead of the "Lib" abbreviation.
The PR does not remove the old names, only adds the new normalized ones to faciliate a transition period.
This way, we can explicitly signal if a test requires the presence
of macOS SDK to build. For instance, when testing our in-house
MachO linker for correctly linking Objective-C, we require the
presence of the SDK on the host system, and we can enforce this
with `-Denable-macos-sdk` flag to `zig build test-standalone`.