Time for a recap!
I am personally not bothering with moving imp.get_tag() and imp.get_magic() to importlib._bootstrap as the amount of C code required to support the public C API is not worth it. If someone else once to do the work then by all means attach a patch.
I still need to port imp.find_module() (and the various constants) and will base it off of Eric's code.
NullImporter will get ported once issue #14605 (exposing the import machinery) lands.
get_suffixes() will also get ported, but that is a little bit more involved as I need to change how _DynLoadFiletab works by only storing the file extensions and not the other fluff (which is the same for all OSs).
After all of that is done then I will expose some API in importlib to replace find_module() and load_*() functions and then deprecate them (see issue #14551 for an ongoing discussion the possible API).