This removes the warning when using default_options without fallback
kwarg completely because a subproject does not know if the main project
has an implicit fallback or not, so it could set default_options even if
not fallback is available at all.
Fixes: #9278
It should build the fallback subprject with default_library=static and
override the dependency for both static=True and static kwarg not given.
Fixes: #8050.
The dependency lookup is a lot of complex code. This refactor it all
into a single file/class outside of interpreter main class. This new
design allows adding more fallbacks candidates in the future (e.g. using
cc.find_library()) but does not yet add any extra API.