(#15596) External facts dirs configurable via environment variable

Eliminate command line options for setting external fact directories.
Configure external fact directories using the environment variable FACTER_LIB.

Prior to this commit, the external fact directories were either the
platform-specific hard-coded defaults or, using a single directory,
passed in via command line option. This commit alters the functionality
so that it uses an environment variable which can handle multiple paths.

When ENV[FACTER_PATH] is set to empty string, no external fact dir path
will be used. When it is not set, the platform-specific hard-coded defaults
will be used.