Move parsing functions to parsers/ and the specific kernel handling...

...functions to pkg/parsers/kernel, and parsing filters to pkg/parsers/filter. Adjust imports and package references.

This is a sizeable patch but changes no logic -- tests are moved to the right places and all tests still pass, things compile, etc.

I didn't ask before making this but I think it's a start. Other parsers could be grouped in here (buildfile.go...) and it's obvious where parsers would live.

Please don't make me rebase this 40 times. :)