0.7.3: added haskell-platform (for linux) and a lot of fixes

• added plug_check_deps into scala
• fixed logic for DEBUG=yes
• improved mk:
  • clear env dir if configure failed
  • more debug messages
  • ensure that we have a version number before starting the download
  • exit cleanly when failing to download the requested package
• added plugin for haskell-platform (linux
  only)
• fixed test for ls-plugins
• common: make curl fail on HTTP errors
• activate: propagate NV_PATH when creating the subshell environment
• added sbt installation into scala plugin
• fixed rust/rust-prebuilt plugin
• fixed ls-versions for elixir/elixir-prebuilt