go2bp: Add -limit and -skip-tests to more easily limit imported modules

I wrote these flags primarily for build-tools:external/kythe, where we
only use a subset of the Go packages defined in the project.

-limit allows defining the entry points to keep in the Android.bp, and
the utput will omit everything except those entry points and their
dependencies. This allows the kythe Android.gen.bp file to drop from 195
modules to 31 using two flags, without having to manually exclude the
other 160 packages.

-skip-tests does as you'd expect, though ideally it wouldn't be
necessary. We didn't import all of the test-only dependencies for kythe
into the source tree, so we're not able to build them.

Change-Id: I49e9f12dac53a74c6e05725ee41fb7c0ed8b0cbb