Fix modules path for XCBuild

[neonichu]

When using XCBuild, we don't segment Swift modules into a "Modules" subdirectory, so we have to special case this during installation.

This should fix the same issue as the revert in #7173

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test package compatibility

[neonichu]

Traceback (most recent call last):
  File "/home/build-user/swiftpm/Utilities/bootstrap", line 828, in <module>
    main()
  File "/home/build-user/swiftpm/Utilities/bootstrap", line 65, in main
    args.func(args)
  File "/home/build-user/swiftpm/Utilities/bootstrap", line 397, in install
    install_swiftpm(prefix, args)
  File "/home/build-user/swiftpm/Utilities/bootstrap", line 437, in install_swiftpm
    install_dylib(args, "PackageDescription", dest, ["PackageDescription", "CompilerPluginSupport"])
  File "/home/build-user/swiftpm/Utilities/bootstrap", line 452, in install_dylib
    if re.match("macosx-", args.cross_compile_hosts):
  File "/usr/lib/python3.8/re.py", line 191, in match
    return _compile(pattern, flags).match(string)
TypeError: expected string or bytes-like object

[neonichu]

@swift-ci please test

[neonichu]

@swift-ci please test package compatibility

[neonichu]

@swift-ci please test windows