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

@@ -489,22 +489,23 @@ class DebMirrorLikeSuiteContentLoader(SuiteContentLoader):
                                             binary_dir,
                                             'Packages')
                     filename = possibly_compressed(filename)
-                    udeb_filename = os.path.join(basedir,
-                                                 component,
-                                                 "debian-installer",
-                                                 binary_dir,
-                                                 "Packages")
-                    # 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,
-                                             packages)
+                    if getattr(self._base_config, 'has_udebs', 'yes') == 'yes':
+                        udeb_filename = os.path.join(basedir,
+                                                     component,
+                                                     "debian-installer",
+                                                     binary_dir,
+                                                     "Packages")
+                        # We assume the udeb Packages file is present if the
+                        # regular one is present
+                        udeb_filename = possibly_compressed(udeb_filename)
+                        self._read_packages_file(udeb_filename,
+                                                 arch,
+                                                 suite.sources,
+                                                 packages)
                 # create provides
                 provides = create_provides_map(packages)
                 binaries[arch] = packages