distro_info: match changelog suite names with their series

Debian packages conventionally target 'unstable' in their
debian/changelog distribution field, but the series data (the
distro-info CSVs) only knows codenames: the suite is the alias
'unstable' of the series 'sid', a mapping the debian-distro-info tool
resolves internally without exposing it in its data.

Add a per-dist suite_aliases reference-data key (debian: unstable ->
sid), with two helpers on top: resolve_suite_alias, identifying a
changelog suite name with its series codename and the dist that
codename belongs to, and series_suite_alias, the inverse direction.
The two names identify the same series.

data/distro_info.yml

@@ -28,6 +28,12 @@
 ##                   series (`<series>-updates`, ...). Deliberately not the
 ##                   `pockets` key: that one is the *search order* of pull,
 ##                   where backports must not fold in.
+##   suite_aliases:  the changelog suite names that alias a series
+##                   codename: Debian packages conventionally target
+##                   'unstable' where the series data carries 'sid'.
+##                   Mapped suite name -> series codename; the two
+##                   names identify the same series, and the selector
+##                   offers the aliased entry as '<suite> (<series>)'.
 ##   build_profiles: the vendor's default DEB_BUILD_PROFILES (Ubuntu
 ##                   activates derivative.ubuntu noudeb, Debian none),
 ##                   mirroring what Dpkg::BuildProfiles resolves when the
@@ -47,6 +53,10 @@ dist:
       - updates
       - security
       - proposed-updates
+    # Debian changelogs conventionally target 'unstable'; the series data
+    # knows the same series as 'sid'.
+    suite_aliases:
+      unstable: sid
     sections:
       # Valid Section values for debian/control: the Debian policy section
       # list unioned with the sections observed in the live Ubuntu archive.