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This prevents the jenv plugin from messing up the PATH and putting `user/local/bin` back at the front of the PATH, undoing any careful setup done before enabling this plugin.

recent `brew install jenv` installs to `/usr/local/bin/jenv`, auto-discover it for recent brew installs to avoid secondary, slower search

`brew --prefix jenv` doesn't ensure jenv is installed so we have to recheck if the
bin folder is still there.

Initializes jenv and provides the jenv_prompt_info funtion to add
Java version information to prompts.  This function is stubbed in
prompt_info_functions script to allow it to be safely called
regardless of whether or not the jenv plugin is loaded.

It also splits detection of the plugin/versions directory and bin directory
to suppport the way Homebrew splits the jenv bin and data directories