Remove the spack.architecture module (#25986)

The `spack.architecture` module contains an `Arch` class that is very similar to `spack.spec.ArchSpec` but points to platform, operating system and target objects rather than "names". There's a TODO in the class since 2016:

abb0f6e27c/lib/spack/spack/architecture.py (L70-L75)

and this PR basically addresses that. Since there are just a few places where the `Arch` class was used, here we query the relevant platform objects where they are needed directly from `spack.platforms`. This permits to clean the code from vestigial logic.

Modifications:
- [x] Remove the `spack.architecture` module and replace its use by `spack.platforms`
- [x] Remove unneeded tests

var/spack/repos/builtin.mock/packages/multimethod/package.py

@@ -5,7 +5,7 @@
 
 from six import string_types
 
-import spack.architecture
+import spack.platforms
 from spack import *
 from spack.pkg.builtin.mock.multimethod_base import MultimethodBase
 
@@ -83,7 +83,7 @@ def has_a_default(self):
     #
     # Make sure we can switch methods on different target
     #
-    platform = spack.architecture.platform()
+    platform = spack.platforms.host()
     targets = list(platform.targets.values())
     if len(targets) > 1:
         targets = targets[:-1]

var/spack/repos/builtin/packages/arm/package.py

@@ -43,7 +43,7 @@
 
 
 def get_os():
-    spack_os = spack.architecture.platform().default_os
+    spack_os = spack.platforms.host().default_os
     return _os_map.get(spack_os, 'RHEL-7')
 
 

var/spack/repos/builtin/packages/gcc/package.py

@@ -10,7 +10,7 @@
 
 import llnl.util.tty as tty
 
-import spack.architecture
+import spack.platforms
 import spack.util.executable
 from spack.operating_systems.mac_os import macos_sdk_path, macos_version
 
@@ -324,7 +324,7 @@ def filter_detected_exes(cls, prefix, exes_in_prefix):
                 continue
             # Filter out links in favor of real executables on
             # all systems but Cray
-            host_platform = str(spack.architecture.platform())
+            host_platform = str(spack.platforms.host())
             if os.path.islink(exe) and host_platform != 'cray':
                 continue
 

var/spack/repos/builtin/packages/sqlite/package.py

@@ -2,12 +2,11 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
 import os
 import re
 from tempfile import NamedTemporaryFile
 
-from spack import architecture
+import spack.platforms
 
 
 class Sqlite(AutotoolsPackage):
@@ -168,9 +167,8 @@ def libs(self):
         return find_libraries('libsqlite3', root=self.prefix.lib)
 
     def get_arch(self):
-        arch = architecture.Arch()
-        arch.platform = architecture.platform()
-        return str(arch.platform.target('default_target'))
+        host_platform = spack.platforms.host()
+        return str(host_platform.target('default_target'))
 
     def configure_args(self):
         args = []

var/spack/repos/builtin/packages/upcxx/package.py

@@ -7,7 +7,7 @@
 
 
 def cross_detect():
-    if spack.architecture.platform().name == 'cray':
+    if spack.platforms.host().name == 'cray':
         if which('srun'):
             return 'cray-aries-slurm'
         if which('aprun'):