Add experimental EPREFIX/EROOT support to the config and vartree

Add experimental EPREFIX/EROOT support to the config and vartree
classes, and use it in ResolverPlayground to emulate a prefix-like
environment. This fixes ResolverPlayground so that it doesn't have
to abuse the --root and --root-deps options in order to create a
testing environment. Instead it simply creates a temporary EPREFIX.
WARNING: EPREFIX/EROOT support is experimental and may be incomplete
for cases in which EPREFIX is non-empty.