Use 5.18.2 as default Perl version if no perl package in channels.
Use 5.18.2 as default Perl version if no perl package in channels.

Also fix rare issue where MetaCPAN `archive` field for package
includes a directory. Now we call `basename`.

dan-blanchard/conda-build,takluyver/conda-build,ilastik/conda-build,mwcraig/conda-build,takluyver/conda-build,shastings517/conda-build,frol/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,sandhujasmine/conda-build,mwcraig/conda-build,dan-blanchard/conda-build,rmcgibbo/conda-build,dan-blanchard/conda-build,mwcraig/conda-build,rmcgibbo/conda-build,sandhujasmine/conda-build,frol/conda-build,frol/conda-build,ilastik/conda-build,shastings517/conda-build,takluyver/conda-build,ilastik/conda-build,shastings517/conda-build