More support for optional home-unit

Closed Sylvain Henry requested to merge hsyl20/ghc:hsyl20/optional-home-unit into master

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.

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