Skip processing install plan when there is nothing to install

Of course processing an empty install plan doesn't do very much
but did give some slightly confusing messages at higher verbosity
levels.
Bootstrap over HTTPS.
Rewrite documentation for `Distribution.License`.

* Fix copyright field.
* List valid versions for each license.
* Remove descriptions for most licenses; just link to them instead.
* Explain why BSD4 should be avoided.
* State that public domain is not a license.
* Link to user guide.
Eliminate redundant wording.
Merge branch 'license-docs' of https://github.com/ianbollinger/cabal