After rereading the Python import documentation, the correct answer to my original problem is:
To ensure that b imports a from its own package its just enough to write the following in the b:
import a
Here is the quote from the docs:
The submodules often need to refer to each other. For example, the
surround module might use the echo module. In fact, such references
are so common that the import statement first looks in the containing
package before looking in the standard module search path.
Note: As J.F. Sebastian suggest in the comment below, use of implicit imports is not advised, and they are, in fact, gone in Python 3.