Description of problem:
In case there is no group_gz metadata available in a repository, dnf downloads uncompressed plain xml variant - xz or zck files are not used at all.

How reproducible:
100%

Steps to Reproduce:
1. have a repository without group_gz metadata defined. E.g. here is an excerpt from F32 repomd.xml - only plain .xml, .xz and .zck metadata are used (no .gz):

-------- 8< --------
  <data type="group">
    <checksum type="sha256">961f955de96f1489e49012cfba125d771d2b4dce6cb31678aea644dffcfb3935</checksum>
    <location href="repodata/961f955de96f1489e49012cfba125d771d2b4dce6cb31678aea644dffcfb3935-comps-Everything.x86_64.xml"/>
    <timestamp>1587588985</timestamp>
    <size>1772199</size>
  </data>
  <data type="group_xz">
    <checksum type="sha256">e07100d0ba13f87849cd17d7b9e2a2a6033310983042c8cf5461141009436373</checksum>
    <location href="repodata/e07100d0ba13f87849cd17d7b9e2a2a6033310983042c8cf5461141009436373-comps-Everything.x86_64.xml.xz"/>
    <timestamp>1587594018</timestamp>
    <size>259956</size>
  </data>
  <data type="group_zck">
    <checksum type="sha256">b7e1566c87075299c4a199737b341e7d15fbf1a4d49c4c2cbe43368437b02c47</checksum>
    <open-checksum type="sha256">e07100d0ba13f87849cd17d7b9e2a2a6033310983042c8cf5461141009436373</open-checksum>
    <header-checksum type="sha256">70457131f086c5ecd94393551d4d1ad2030a13ecd61ac6ef7d6973981c1f62c7</header-checksum>
    <location href="repodata/b7e1566c87075299c4a199737b341e7d15fbf1a4d49c4c2cbe43368437b02c47-comps-Everything.x86_64.xml.zck"/>
    <timestamp>1587594156</timestamp>
    <size>477385</size>
    <open-size>259956</open-size>
    <header-size>1200</header-size>
  </data>
-------- 8< --------

2. synchronize dnf repositories metadata cache (dnf makecache)


Actual results:
The uncompressed comps.xml is downloaded:
$ ls -la /var/cache/dnf/fedora-558931b5e76b51a7/repodata/*comps*
-rw-r--r--. 1 root root 1772199  5. lis 09.16 /var/cache/dnf/fedora-558931b5e76b51a7/repodata/961f955de96f1489e49012cfba125d771d2b4dce6cb31678aea644dffcfb3935-comps-Everything.x86_64.xml


Expected results:
Compressed file is used.


Additional info:
rpmfusion repositories workaround it by using comps.xml.xz in group_gz metadata.