Always insert a code line of the form:  our $AUTHORITY = ...;

This makes both code paths insert identical code, and also eliminates the
"used only once" warning that arises from "perl lib/Foo.pm".

This will also let me rewrite Dist::Zilla::Plugin::Test::Compile to test packages by loading the .pm files directly. which fixes some fiddly edge cases with module names containing unicode. (I'll enter a x_breaks field into DZPTC's metadata, via the [Conflicts] plugin.)