Add proper platform support
Related to #36 Presently, we tend to enable platform-specific config with an overlay. There are a variety of problems identified with this approach: 1. There is no convention for which overlay to use for platform- specific config. This complicates the ecosystem. 2. In general, a program is always compiled "for some platform" but in practice a user may forget the necessary overlay. `require-overlays` can catch this misconfig, but A) it has to be opted into in the atpkg, and B) there is no good way to default to the running platform, which is the sane behavior. 3. Currently, tools tend to conditionally-compile platform-specific code. The effect of this is to complicate bootstrapping, as a Linux binary *cannot* perform some OSX behavior (and vice versa) because the necessary code is not present in the executable. 4. This is not scaleable to cross-compilation (iOS for example is Coming Soon
™but can't be supported in this architecture) To work around these problems, we introduce a `Platform` enum, to replace `PlatformPaths`. `Platform` is a runtime technology, allowing for a Linux binary to reason about what the behavior would be on OSX etc. Internally, we have three "platforms": * `hostPlatform`, the platform on which `atbuild` is currently executing * `targetPlatform`, the platform for which we are compiling. By default this is the `hostPlatform` * `buildPlatform`, the platform where `swift-build-tool` will run. This is usually the `hostPlatform`, but if we are bootstrapping it is the `targetPlatform` instead. The user can override the `targetPlatform` by the use of `--platform foo`. `linux` and `osx` are supported. `mac` is supported as an alias of `osx`. The primary effect of a platform is to scope tool-specific behavior (e.g., target=OSX uses the OSX SDK, host=Linux uses a linux path for the toolchain, etc). In addition to the tool-specific behavior, we enable overlays for the target platform: * `atbuild.platform.linux` on Linux * `atbuild.platform.osx` and `atbuild.platform.mac` on OSX This allows packages to reliably perform per-platform configuration in the overlays. Critically, some platform overlay is reliably active, so users in most cases will not have to `--use-overlay` to get proper platform behavior. DEPRECATION WARNING: We believe the `swiftc-path` key is no longer required, as the functionality used can be achieved either by `--toolchain` or `--platform`. Therefore, I'm adding a warning to users that we intend to remove it and to try these features instead. We need to put out a release with these features (and the warning) before I'm happy to remove it. In particular, we use it inside atbuild/atpkg, and removing it immediately would break bootstrapping, so let's give it a little time before we tear it out. We should remove it from the documentation though.
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