Add ":valid-package-names:" option to automodsumm/automodapi - #1808

@eteq - is there any reason to add it to 0.3.0 though? If it'll only be useful for 0.3.1, maybe we can merge after 0.3.0?

@astrofrog - technically this is a new feature in automodsumm, so if we strictly adhere to "bugfixes only", it has to wait for 0.4. But given that this is pretty deep in the weeds, I'm filling to consider it a "bugfix" if you are :)

Since minor versions can include documentation fixes, and this code should only affect the docs, as far as I'm concerned this is fine to include in 0.3.1 along with whatever pages use it. I think we should just avoid include any non-critical code in the 0.3.0 release now the RC is out.

Alright, good enough for me. I'll close this in favor of #1801, then.