core: add an Arch type (MR 2252)

Move pmb/parse/arch.py over to core and refactor it as an Arch type,
similar to how Chroot was done. Fix all the uses (that I can find) of
arch in the codebase that need adjusting.

The new Arch type is an Enum, making it clear what architectures can be
represented and making it much easier to reason about. Since we support
~5 (kinda) different representations of an Architecture (Alpine, Kernel,
target triple, platform, and QEMU), we now formalise that the Alpine
format is what we represent internally, with methods to convert to any
of the others as-needed.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/helpers/package.py

@@ -10,6 +10,7 @@ See also:
 """
 import copy
 from typing import Any, Dict
+from pmb.core.arch import Arch
 from pmb.core.context import get_context
 from pmb.helpers import logging
 import pmb.build._package
@@ -83,7 +84,7 @@ def get(pkgname, arch, replace_subpkgnames=False, must_exist=True):
     # Find in APKINDEX (other arches)
     if not ret:
         pmb.helpers.repo.update()
-        for arch_i in pmb.config.build_device_architectures:
+        for arch_i in Arch.supported():
             if arch_i != arch:
                 ret = pmb.parse.apkindex.package(pkgname, arch_i, False)
             if ret: