Untangle REPL code from HTM code, autoload more responsibly

It was non-trivial to get user.clj to load cljx code, since Clojure loads user.clj long before running the project's prep-tasks

project.clj change commentary:
- Rely on cljx's repl-middleware to load cljx files, rather than falling back to "target/classes"
- Stopped running "cljx once" all the time. Only use it on install.
- Remove generated paths from :source-paths. It's sufficient for this code to sit in target/classes. There were lots of "duplicate file" errors on install, and this was why.
- cljx folder is still included in :source-paths because some tools use it for go-to-definition, and because there's a single .clj file in src
- Moved :cljx and :cljsbuild spec out of profiles. It's good enough to scope the plugin to the profile.
- "lein do check, install" is no longer right. "lein do install, check" or "lein do cljx, check" are valid.

Also, a better fix to the recent cljs :import regression. Import classes via :require in cljs (though Clojure requires :import)