[bldr-build] Group all internal and public helper functions.
[bldr-build] Group all internal and public helper functions.

This is an attempt to further communicate which functions make up the
public/supported API of `bldr-build` to Plan authors in their `plan.sh`
files. The adopted convention is to prefix all internal/private
functions with an underscore (i.e. `_find_system_commands()`).

Note that for purely technical reasons (namely having the early
`on_exit` trap logic work), the private/internal helper functions appear
before the public ones.

habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat