To allow us to implement case-insensitivity in package names given on the
command line. We lookup in the package index for packages with the same name
case-insensitively. If there is no exact match exact case-sensitively and
there are more than one packages matching case-insensitively then it aborts
with a message listing the matches. This should not often happen since within
any single HackageDB server, we can check that packages names are unique
case-insensitively but it's possible to get ambiguities if cabal-install has
been configured to use multiple repos.