Revert "bsdinstall: separate out dist selection in prep for pkgbase support"

Firstly, my review comments were not addressed and instead totally
ignored. Secondly, and a more valid justification for the revert, this
completely breaks the installer, since selectdists isn't installed.
Given the blatant lack of testing, back out this commit until it has
actually been tested and review comments taken on board so that the
installer actually works.

This reverts commit 009d3f66cb5f0cf3f1d353f311d3a6878b2a534e.