Updating beta versions might lead to build failures due to remainings of a previous version

[toco]

After updating to Xcode 7 beta 5 I had following build issue:

Module file was created by an older version of the compiler; rebuild 'Security' and try again: /Applications/Xcode-7.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/watchsimulator/i386/Security.swiftmodule

Security.swiftmodule was installed with a previous beta version but does not exist in the beta 5.
xcode-install uses ditto to install Xcode, which merges files from the source into existing directories at the destination path.
If a new (beta) version of Xcode doesn't contain files which were present in the previous version, the old files will still be there after installing the update.

Consider to automatically uninstall previous version at the destination path or to rename them before the install. If the install succeeded the old version can be deleted, but can still be used if the install failed.

[neonichu]

Until #4 is implemented, I believe the correct behaviour would be to refuse installation if there's an existing installation at the target path. This will allow the user to proceed as they see fit.