Fixed custom SQL query rewriting in PodsData->build().

I noticed that whenever I passed a custom `$params['sql']` to `PodsData->build()`, it would simply be ignored. I dug into the code and fixed it up. The handling is backwards now: instead of treating a custom query as a 'special case', the default query is now the special case.

I tested it with some of the regular queries used by Pods and they all still seem to work fine.

Note: I borked up the tabs again, sorry for the unclear diffs in the commit. :confused: I tried a [compare view](https://github.com/MattiasBuelens/pods/compare/dc058486caa2c4fe906893e61350606e215c0616...2.1.x), but it still doesn't properly track all moves and changes.