[3.8] bpo-37409: fix relative import with no parent (GH-14956) (GH-15913)

[3.8] bpo-37409: fix relative import with no parent (GH-14956) (GH-15913)

Relative imports use resolve_name to get the absolute target name,
which first seeks the current module's absolute package name from the globals:
If __package__ (and __spec__.parent) are missing then
import uses __name__, truncating the last segment if
the module is a submodule rather than a package __init__.py
(which it guesses from whether __path__ is defined).

The __name__ attempt should fail if there is no parent package (top level modules),
if __name__ is '__main__' (-m entry points), or both (scripts).
That is, if both __name__ has no subcomponents and the module does not seem
to be a package __init__ module then import should fail..
(cherry picked from commit 92420b3e679959a7d0ce875875601a4cee45231e)

Co-authored-by: Ben Lewis <benjimin@users.noreply.github.com>