Publish: check and clean `dist/` dir if already pre-populated or dirty

[sohkai]

To avoid publishing too many files (e.g. old source maps, bundles, or even artifacts), it would be nice if the publish command checked the stat of dist/ before going further in the build process.

For more context, see https://github.com/aragon/buidler-aragon/pull/121#issuecomment-609401958.