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/run_core.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 import fcntl
 from pmb.core import get_context
+from pmb.core.arch import Arch
 from pmb.types import PathString, Env
 from pmb.helpers import logging
 import os
@@ -35,6 +36,8 @@ def flat_cmd(cmds: Sequence[Sequence[PathString]], working_dir: Optional[Path]=N
     # Merge env and cmd into escaped list
     escaped = []
     for key, value in env.items():
+        if isinstance(value, Arch):
+            value = str(value)
         escaped.append(key + "=" + shlex.quote(os.fspath(value)))
     for cmd in cmds:
         for i in range(len(cmd)):