Fixing #4268 - manifests always imported

The problem is that the environment list gets cleared
when Settings#set_value is called, and it was being
called every time Settings#use was called, which is
more often than obvious but especially if reporting
is enabled.

Previously we ignored noop when running inside of Settings,
and this essentially adds that back in, so we can remove
the special noop behaviour in Settings itself.

Signed-off-by: Luke Kanies <luke@puppetlabs.com>