[] Install system packages using system-packages-install

[] Install system packages using system-packages-install

:ensure-system-package was installing packages by running
system-packages-get-command via async-shell-command. This meant that
system-packages-use-sudo wasn't being honoured.

This patch makes :ensure-system-package use system-packages-install
for all cases, except where a custom install command is supplied, in
which case async-shell-command is used.

This issue was introduced in 9f034a0bcfdd8c4 [https://github.com/jwiegley/use-package/issues/673], as a fix for
[https://github.com/jwiegley/use-package/issues/661]. Prior to that commit, system-packages-use-sudo was being
honoured.

This patch also fixes a bug where a cons containing a lone symbol in a
list of conses causes nil to used as the package to install.
GitHub-reference: fix https://github.com/jwiegley/use-package/issues/720
Copyright-paperwork-exempt: yes