Removes dependency on apt python lib.

It seemed far too much complicated find the proper way to deal with
apt Python library under the snap environment.

Opted for running the system apt command without the LD_*
customisations, similar to what an installer does when chroot into the
taget system.

system_setup/server/controllers/configure.py

@@ -17,8 +17,6 @@ import shutil
 import logging
 import re
 from typing import Optional, Tuple, List
-import apt
-import apt_pkg
 
 from subiquity.common.errorreport import ErrorReportKind
 from subiquity.common.types import ApplicationState
@@ -174,27 +172,36 @@ class ConfigureController(SubiquityController):
         """ Install recommended packages.
         lang is expected to be one single language/locale.
         """
-        packages = await self.__recommended_language_packs(lang, env)
+        packages = await self.__recommended_language_packs(lang)
+        # Hardcoded path is necessary to ensure escaping out of the snap env.
+        aptCommand = "/usr/bin/apt"
         if packages is None:
             log.error('Failed to detect recommended language packs.')
             return False
 
-        cache = apt.Cache()
+        cp = await arun_command([aptCommand,"update"], env=env)
+
+        if cp.returncode != 0:
+            # TODO: deal with the error case.
+            log.error("Failed to update apt cache.\n%s", cp.stderr)
+
+
         if self.app.opts.dry_run:  # only empty in dry-run on failure.
             if len(packages) == 0:
                 log.error("Packages list in dry-run should never be empty.")
                 return False
 
             packs_dir = os.path.join(self.model.root,
-                                     apt_pkg.config
-                                     .find_dir("Dir::Cache::Archives")[1:])
+                                     "var/cache/apt/archives/")
             os.makedirs(packs_dir, exist_ok=True)
             try:
                 for package in packages:
                     # Just write the package uri to a file.
-                    archive = os.path.join(packs_dir, cache[package].fullname)
+                    lcp = await arun_command([aptCommand,"install",package,
+                                              "--simulate"], env=env)
+                    archive = os.path.join(packs_dir, package)
                     with open(archive, "wt") as f:
-                        f.write(cache[package].candidate.uri)
+                        f.write(lcp.stdout)
 
                 return True
 
@@ -206,15 +213,11 @@ class ConfigureController(SubiquityController):
             log.info("No missing recommended packages. Nothing to do.")
             return True
 
-        cache.update()
-        cache.open(None)
-        with cache.actiongroup():
-            for package in packages:
-                cache[package].mark_install()
+        cmd = [aptCommand,"install","-y"] + packages
+        acp = await arun_command(cmd, env=env)
 
-            cache.commit()
+        return acp.returncode==0
 
-        return True
 
     def _update_locale_gen_file(self, localeGenPath, lang) -> bool:
         """ Uncomment the line in locale.gen file matching lang,