Throw exception if subpackage function can't be found
Right now, pmbootstrap just accepts that a subpackage function is missing if it cannot be found: https://gitlab.com/postmarketOS/pmbootstrap/-/blob/44781163795763b44b2f289acf38c0a1c23d7d2e/pmb/parse/_apkbuild.py#L261-272
This can result in confusing and difficult to debug issues, like the "unreferenced file" problems in pmaports!4714 (closed), which happened because pmbootstrap couldn't find the function for the sway subpackage (see !2267 (merged)). As such, like the comment in the code above suggests, we should throw an exception if the subpackage function cannot be found and add some sort of special handling for subpackages where it doesn't have to be found (like doc, dev, dbg, and so on subpackages).