More support for optional home-unit

This is an independent minor refactoring extracted from !6748 (closed) to make it simpler:

Commit message:

This is a preliminary refactoring for #14335 (supporting plugins in cross-compilers). In many places the home-unit must be optional because there won't be one available in the plugin environment (we won't be compiling anything in this environment). Hence we replace "HomeUnit" with "Maybe HomeUnit" in a few places and we avoid the use of "hsc_home_unit" (which is partial) in some few others.