[Experimental][Build] Add support for building gyb files

[aciidgh]

This will generate commands to build gyb files if they're present in a
Swift target. Currently, there are no customization available and
the gyb compiler will be looked up in the environment variable "GYB".

I tested this patch with SwiftSyntax on macOS, using this manifest file:

// swift-tools-version:4.0

import PackageDescription

let package = Package(
    name: "SwiftSyntax",
    products: [
        .library(name: "libSwiftSyntax", type: .dynamic, targets: ["SwiftSyntax"]),
    ],
    targets: [
        .target(
            name: "SwiftSyntax", path: ".")
    ]
)

[aciidgh]

@ddunbar I hacked up this gyb support but I am not sure if this is something we should actually land. What do you think?

[aciidgh]

/cc @harlanhaskins

[harlanhaskins]

I’ll be honest, I really appreciate you taking the time for this, but I’m generally opposed to gyb leaking outside the swift repo. I wouldn’t want developers attempting to make use of these APIs by copying gyb from apple/swift...

[ddunbar]

Yeah, I don't want to do this. If we want something like this, it should come via extensible build tools.

[harlanhaskins]

There are a lot of code generators out there. Maybe a customizable preprocessing step for certain build files?

[aciidgh]

Thats fair! Instead of supporting this with extensible build tools, maybe we can add support for code generators in general? I think that may be an easier proposal than extensible build tools but OTOH will provide less flexibility.