We intentionally try to enlarge the configuration space only as little
as needed, thus we partition the space for the cabal solver into

 - xhtml-3000.2.2   supports base  < 4.11 & cabal-version: 1.10
 - xhtml-3000.2.2.1 supports base >= 4.11 & cabal-version: 2.0

as xhtml-3000.2.2 and xhtml-3000.2.2.1 are not distinguishable except
for their version number.