Rename option to switch gather backend

Calling it gather_backend is similar to repoclosure_backend we already
have. It's also more obvious what it does.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -531,10 +531,10 @@ Options
         * With ``greedy_method = "build" ``pkg-b-provider-1`` and
           ``pkg-b-provider-2`` will be pulled in.
 
-**dnf_gather** = False
-    (*bool*) -- When set to ``True``, DNF backend is used instead of YUM. This
-    changes the entire codebase doing dependency solving, so it can change the
-    result in unpredictable ways.
+**gather_backend** = ``yum``
+    (*str*) -- Either ``yum`` or ``dnf``. This changes the entire codebase
+    doing dependency solving, so it can change the result in unpredictable
+    ways.
 
     Particularly the multilib work is performed differently by using
     ``python-multilib`` library. Please refer to ``multilib`` option to see the

pungi/checks.py

@@ -505,7 +505,11 @@ def _make_schema():
             },
             "gather_prepopulate": {"$ref": "#/definitions/str_or_scm_dict"},
             "gather_source_mapping": {"type": "string"},
-            "dnf_gather": {"type": "boolean", "default": False},
+            "gather_backend": {
+                "type": "string",
+                "enum": ["yum", "dnf"],
+                "default": "yum",
+            },
 
             "pkgset_source": {
                 "type": "string",

pungi/phases/gather/methods/method_deps.py

@@ -119,10 +119,11 @@ def resolve_deps(compose, arch, variant):
     tmp_dir = compose.paths.work.tmp_dir(arch, variant)
     cache_dir = compose.paths.work.pungi_cache_dir(arch, variant)
     # TODO: remove YUM code, fully migrate to DNF
-    if compose.conf.get("dnf_gather", False):
-        get_cmd = pungi_wrapper.get_pungi_cmd_dnf
-    else:
-        get_cmd = pungi_wrapper.get_pungi_cmd
+    backends = {
+        'yum': pungi_wrapper.get_pungi_cmd,
+        'dnf': pungi_wrapper.get_pungi_cmd_dnf,
+    }
+    get_cmd = backends[compose.conf['gather_backend']]
     cmd = get_cmd(pungi_conf, destdir=tmp_dir, name=variant.uid,
                   selfhosting=selfhosting, fulltree=fulltree, arch=yum_arch,
                   full_archlist=True, greedy=greedy_method, cache_dir=cache_dir,