suiteloader: add configuration option whether to load udebs.

udebs do not have symbol versioned shlibdeps, thus it means that when
there is glibc in proposed, any packages built in proposed get a udeb
depends on the strictly higher version of glibc in proposed. This
leads to entaglement of the $world with glibc transition. Similar
story with glib, openssl, etc.

By setting the new configuration option to HAS_UDEBS = no, one can
make suiteloader not look at udebs, and thus use britney even when
installability of udebs is not required.

britney.conf

@@ -44,6 +44,9 @@ BREAK_ARCHES      =
 # if you're in this list, you are a new architecture
 NEW_ARCHES        =
 
+# should udebs be looked at?
+HAS_UDEBS         = yes
+
 # priorities and delays
 MINDAYS_LOW       = 0
 MINDAYS_MEDIUM    = 0

britney2/inputs/suiteloader.py

@@ -489,6 +489,11 @@ class DebMirrorLikeSuiteContentLoader(SuiteContentLoader):
                                             binary_dir,
                                             'Packages')
                     filename = possibly_compressed(filename)
+                    self._read_packages_file(filename,
+                                             arch,
+                                             suite.sources,
+                                             packages)
+                    if getattr(self._base_config, 'has_udebs', 'yes') == 'yes':
                         udeb_filename = os.path.join(basedir,
                                                      component,
                                                      "debian-installer",
@@ -497,10 +502,6 @@ class DebMirrorLikeSuiteContentLoader(SuiteContentLoader):
                         # We assume the udeb Packages file is present if the
                         # regular one is present
                         udeb_filename = possibly_compressed(udeb_filename)
-                    self._read_packages_file(filename,
-                                             arch,
-                                             suite.sources,
-                                             packages)
                         self._read_packages_file(udeb_filename,
                                                  arch,
                                                  suite.sources,