Refactor CmdInstall

CmdInstall.installAction is ~300 lines long and full of nested scopes
and ad-hoc logic. This change hopes to make it more readable and
understandable.

- Lift withProject and withoutProject out of installAction and split
  their relative concerns. E.g. not parsing URIs is installAction's
  concern not withProject's (which would just return a constant []).
- Split an intermediate step into a separate function, resolveTargetSelectorsInProjectBaseContext.
- Reuse withGlobalConfig and specFromPkgId (renamed from pidPackageSpecifiers).
- Avoid trying withProject a second time in case no target is specified.
- Fix a bug introduced in 802a326 where
  establishProjectBaseContext is called in a non-project setting. Also
  simplify its original implementation by moving the change into
  withProject rather than calling establishProjectBaseContext a second
  time.
- Document the interaction between cabal v2-install and local configuration
  and add few comments.