[bldr-build] Add more dependencies into `_find_system_commands`.
[bldr-build] Add more dependencies into `_find_system_commands`.

There are 2 primary motivating reasons for this change:

1. Several necessary system commands were missing and would lead to late
   failure of the program, most sadly a failure to gpg-sign the package
   after it was successfully built. This is a bad user experience as it
   is a very expensive waste of user time.
2. The desire to communicate which parts of `bldr-build` are intended to
   be used by Plan authors and which parts are considered part of the
   internal/private API. In the case of the global variables that are
   set for the command dependencies (i.e. `$tar_cmd`), it seems useful
   to communicate the fact that no Plan user should directly use these
   variables--if they rely directly on gpg, then the Plan should add
   an explicit dependency on `chef/gpg` or similar. A comment further
   elaborating on this is added to the `_find_system_commands`
   function documentation.

be-plans/be,be-plans/be,be-plans/be,be-plans/be