ignore current directory in collect-installed-modules

ExtUtils::Installed searches modules recursively from `@INC` directories, so it takes much CPU and times if it runs at `/`, `~/`, and the like.
I think it shoud ignore "current directory".
https://github.com/hirose31/list-installed-perl-modules/blob/81b5cf6194e5ed748e69f847db60d5c4ee96a61b/list-installed-perl-modules#L30
anyway, modules on irregular path (ex. installed by `cpanm -l local ...`) couldn't open by `perldoc` command.