Instead of expanding XIPs on its current directory, a temporal directory is used. The main advantage of this approach is that, because the temporal directory is created on the same volume where Xcode will be installed, the installation is vastly improved if the XIP is on a different volume.

- https://nshipster.com/temporary-files/
- The implementation of the `temporalDirectory` function and variable are based on the similar `trashItem`, but without the `@discardableResult` annotation (I cannot think of any case where this URL could to be ignored).

This closes XcodesOrg#178.

…de `Files`

As suggested by @MattKiazyk, to be able to get the expansion directory by simply calling `Current.files.xcodeExpansionDirectory()`.

- The `Files.temporalDirectory()` added on the previous commits is keep even if it's not used outside `Files.xcodeExpansionDirectory()`, to keep each function with a single statement (as the other functions on `Files`).
- I'm not sure there's value on adding the `xcodeExpansionDirectory` variable, but added for consistency with the other functions on `Files`.

…rectory

As suggested by @MattKiazyk, to allow the user to revert to the old behaviour if needed.

- For testing this flag is set to `true`, to use a well-known location when testing.
- The `xcodeExpansionDirectory` function cannot be easily expressed on a single line anymore, so removed the variable added on the previous commit (which wasn’t used anyway).

# Conflicts:
#	Sources/XcodesKit/XcodeInstaller.swift
#	Sources/xcodes/main.swift

# Conflicts:
#	Sources/XcodesKit/XcodeInstaller.swift
#	Sources/xcodes/main.swift
#	Tests/XcodesKitTests/XcodesKitTests.swift

# Conflicts:
#	Sources/XcodesKit/XcodeInstaller.swift
#	Sources/xcodes/App.swift

# Conflicts:
#	Sources/XcodesKit/Environment.swift
#	Sources/XcodesKit/XcodeInstaller.swift
#	Sources/xcodes/App.swift
#	Tests/XcodesKitTests/Environment+Mock.swift
#	Tests/XcodesKitTests/XcodesKitTests.swift

# Conflicts:
#	Sources/xcodes/App.swift

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1.6.0