* Unify the default project with the implicit one

Passing --ignore-project made cabal use `defaultProject` which is not
quite identical to the one used when cabal.project is missing
(`defaultImplifictProjectConfig`). There is no reason the two should be
different.

* Warn if builddir is going to be ignored.

builddir can only be specified from the command line.

Closes: #7941

* Clarify which project option can only be passed from the command line

* Add note about reading the project configuration