snapshots: Improve convert_wgsl and convert_spv legibility.

[jimblandy]

Add a use Targets::* declaration to convert_wgsl and convert_spv, so that each test case fits on a single line, to improve legibility.

I think it's an improvement, but it's obviously a matter of taste. If someone objects, I'm fine with the status quo as well.

[teoxoy]

CI is unhappy because you can't "import" associated consts of a struct.

You'd probably have to do this for each target:

let X = Targets::X;
...

---

We could also add those consts to the struct instead:

const MAIN = Self::SPIRV.bits()
    | Self::METAL.bits()
    | Self::GLSL.bits()
    | Self::HLSL.bits()
    | Self::WGSL.bits();
const DEBUG = Self::IR.bits()
    | Self::ANALYSIS.bits()
    | Self::DOT.bits();

Since most snapshots use all the lang targets.

[jimblandy]

Did I really not even test this before posting the PR? Apparently so.

[jimblandy]

Maybe there are better ways to handle all this.