distro_like: new template variables module

Closes #104

apkg/templatevars/distro_like.py

+"""
+Custom template variable module for apkg adding an enhanced distro variable.
+
+Provides custom template variable `distro_like` that behaves exactly like
+`distro`, except that a `distro_like.match()` also matches against the ID_LIKE
+information from the distro (e.g. CentOS has rhel in there, ...) respecting the
+version as well. As such, version matching should be done only when the derived
+distro's version scheme is compatible (e.g. Ubuntu doesn't match Debian's).
+
+This behaviour does not trigger in all cases, if `-d <distro maybe-version>` is
+passed to apkg and the distro specified that way is different from the one
+inferred from the current host, this new variable will *not* be exposed. This
+gives the packager a chance to detect and handle this case however they see
+fit, e.g. by setting `distro_like` themselves while preventing running a build
+with an untested configuration accidentally.
+
+Enable by adding [[template.variables]] entry to distro/config/apkg.toml:
+
+    [[template.variables]]
+    python_module = "apkg.templatevars.distro_like"
+
+Example usage in rpm spec:
+
+{% if distro_like.match('rhel > 9') %}
+{# this also matches on Alma 9, CentOS 10, Rocky 28, ... #}
+{% endif %}
+"""  # noqa
+
+import distro
+
+from apkg import adistro
+
+
+# called by apkg to get custom variables
+def get_variables(env):
+    distro_var = env['distro']
+    # poor man's .copy()
+    distro_like = adistro.Distro(distro_var.orig, distro.aliases)
+
+    if distro.id.lower() == distro_var.id.lower():
+        like = set(distro_var.names) + set(distro.like().split())
+        distro_like.names = list(like)
+        return {'distro_like': distro_like}
+
+    # Otherwise building for a foreign distro, expose nothing
+    return {}