* cabal-install: extract url scheme checks

Extract a bunch of string equality checks for the URI scheme to
top-level functions.

* cabal-install: refactor and document transport checks

"They're the same picture".  Thus, refactor the *transport supports
https* checks.

* cabal-install: allow Basic authentication in curl transport

Allow the curl transport to use Basic authentication, if and only if
the url scheme is HTTPS (i.e. TLS will be used).  Retain the
existing behaviour (force Digest scheme) for insecure requests.

This change is required to support upcoming hackage-server changes.
The wget transport already supports Basic authentication.

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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>