std.Build: introduce `ConfigHeader.getOutputDir`, small refactor

`std.Build.Step.ConfigHeader` emits a *directory* containing a config
header under a given sub path, but there's no good way to actually
access that directory as a `LazyPath` in the configure phase. This is
silly; it's perfectly valid to refer to that directory, perhaps to
explicitly pass as a "-I" flag to a different toolchain invoked via a
`Step.Run`. So now, instead of the `GeneratedFile` being the actual
*file*, it should be that *directory*, i.e. `cache/o/<digest>`. We can
then easily get the *file* if needed just by using `LazyPath.path` to go
"deeper", which there is a helper function for.

The legacy `getOutput` function is now a deprecated alias for
`getOutputFile`, and `getOutputDir` is introduced.

`std.Build.Module.IncludeDir.appendZigProcessFlags` needed a fix after
this change, so I took the opportunity to refactor it a little. I was
looking at this function while working on ziglang/translate-c yesterday
and realised it could be expressed much more simply -- particularly
after the `ConfigHeader` change here.

I had to update the test `standalone/cmakedefine/` -- it turns out this
test was well and truly reaching into build system internals, and doing
horrible not-really-allowed stuff like overriding the `makeFn` of a
`TopLevelStep`. To top it all off, the test forgot to set
`b.default_step` to its "test" step, so the test never even ran. I've
refactored it to follow accepted practices and to actually, like, work.

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