Sylvain Henry authored Aug 06, 2021 and Marge Bot committed Nov 20, 2021

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.