build: install into the macOS style paths

[compnerd]

Install the module into the macOS style paths which are honoured on both
macOS and non-macOS targets. This simplifies the install rules across
all the targets.

[dan-zheng]

Yay! macOS toolchains are working again!

$ cat tf.swift
import TensorFlow
print(Tensor(1))
import PythonKit
print(PythonObject(1))
$ swift tf.swift
1.0
1

But the macOS REPL is super broken (haven't tested Linux REPL):

$ swift
import PythonWelcome to Swift version 5.2-dev (LLVM b3057cffb6, Swift 196e6963bb).
Type :help for assistance.
  1> import PythonKit
  2> PythonObject(1)
error: repl.swift:2:1: error: use of unresolved identifier 'PythonObject'
PythonObject(1)
^~~~~~~~~~~~


  2> import TensorFlow
  3> Tensor(1)
error: repl.swift:3:1: error: use of unresolved identifier 'Tensor'
Tensor(1)
^~~~~~


  3> import alksdjflasdjfsakldfj
  4> # wait what? line above doesn't fail?

And swift build doesn't work anymore (tested on tensorflow/swift-models):

$ swift build
'TensorFlowModels' swift-models: warning: ignoring duplicate product 'Transformer'
swift-models/Models/ImageClassification/DenseNet121.swift:15:8: error: compiling for macOS 10.13, but module 'TensorFlow' has a minimum deployment target of macOS 10.14: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-02-27-a.xctoolchain/usr/lib/swift/macosx/TensorFlow.swiftmodule/x86_64.swiftmodule
import TensorFlow
       ^
swift-models/Gym/FrozenLake/main.swift:16:12: error: compiling for macOS 10.13, but module 'PythonKit' has a minimum deployment target of macOS 10.14: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-02-27-a.xctoolchain/usr/lib/swift/macosx/PythonKit.swiftmodule/x86_64.swiftmodule
    import PythonKit
           ^
swift-models/Gym/CartPole/main.swift:16:12: error: compiling for macOS 10.13, but module 'PythonKit' has a minimum deployment target of macOS 10.14: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-02-27-a.xctoolchain/usr/lib/swift/macosx/PythonKit.swiftmodule/x86_64.swiftmodule
    import PythonKit
           ^

One step at a time 💪

[dan-zheng]

Merging to unblock progress. validation-test/Python/python_lint.swift can be fixed later.

Edit: holy cow oops, I meant to merge swiftlang/swift#30108 (for which validation-test/Python/python_lint.swift is the only failing test). Oh well.

[dan-zheng]

Oops! CI doesn't seem to pass for this PR:

Step 29/36 : WORKDIR /swift-models
 ---> Running in 5730f0d835f9
Removing intermediate container 5730f0d835f9
 ---> c5348eaee397
Step 30/36 : RUN /swift-tensorflow-toolchain/usr/bin/swift build
 ---> Running in 738ce74bc6c6
Fetching https://github.com/apple/swift-protobuf.git
Fetching https://github.com/kylef/Spectre.git
Fetching https://github.com/kylef/Commander.git
Cloning https://github.com/apple/swift-protobuf.git
Resolving https://github.com/apple/swift-protobuf.git at 1.7.0
Cloning https://github.com/kylef/Commander.git
Resolving https://github.com/kylef/Commander.git at 0.9.1
Cloning https://github.com/kylef/Spectre.git
Resolving https://github.com/kylef/Spectre.git at 0.9.0
[1/150] Compiling SwiftProtobuf BinaryDecodingError.swift
[2/150] Compiling SwiftProtobuf BinaryDecodingOptions.swift
[3/150] Compiling SwiftProtobuf BinaryDelimited.swift
[4/150] Compiling SwiftProtobuf BinaryDecoder.swift
[5/150] Compiling SwiftProtobuf BinaryEncodingError.swift
[6/150] Compiling SwiftProtobuf AnyUnpackError.swift
[7/150] Compiling SwiftProtobuf AnyMessageStorage.swift
[8/150] Compiling SwiftProtobuf BinaryEncodingSizeVisitor.swift
[9/150] Compiling SwiftProtobuf Message+AnyAdditions.swift
[10/150] Compiling SwiftProtobuf Message+BinaryAdditions.swift
[11/150] Compiling SwiftProtobuf Message+JSONAdditions.swift
[14/150] Compiling SwiftProtobuf ZigZag.swift
[15/150] Compiling SwiftProtobuf any.pb.swift
[16/150] Compiling SwiftProtobuf ProtobufMap.swift
[17/150] Compiling SwiftProtobuf SelectiveVisitor.swift
[18/150] Compiling SwiftProtobuf duration.pb.swift
[19/150] Compiling SwiftProtobuf empty.pb.swift
[20/150] Compiling SwiftProtobuf Decoder.swift
[21/150] Compiling SwiftProtobuf DoubleParser.swift
[22/150] Compiling SwiftProtobuf Enum.swift
[23/150] Compiling SwiftProtobuf ProtoNameProviding.swift
[24/150] Compiling SwiftProtobuf ProtobufAPIVersionCheck.swift
[25/150] Compiling SwiftProtobuf SimpleExtensionMap.swift
[26/150] Compiling SwiftProtobuf StringUtils.swift
/swift-models/Gym/CartPole/main.swift:18:12: error: no such module 'Python'
    import Python

I wonder if this is a regression or not? cc @compnerd