Add imports property to py_library rules

This is to avoid having it hardcoded in a fork of the
py_library rule.

Most import attributes should just be set to ".", but
our previous solution always hardcoded it to ".." instead,
for ndkstubgen. ndkstubgen uses pkg_path: "ndkstubgen",
i.e., it set pkg_path to the name of the folder that
contained the Android.bp file. In this specific scenario,
imports = ".." works. Recreate that behavior here as well,
because we don't handle pkg_path properly yet.

Fixes: 233081071
Test: build/bazel/ci/bp2build.sh
Change-Id: Ib5e6a8edf428c74d4b5947f0ec53a2151001367a