wixl: use a versionized installation directory
    
    It turned out that not only the current MSI broke the "component rule",
    but also that our files are not versionized correctly. Windows Installer
    applies some file versioning rules before replacing a file
    http://msdn.microsoft.com/en-us/library/aa368599%28v=vs.85%29.aspx
    
    Since msitools doesn't extract version from files and populate the Version
    field of the File table, it "usually" keep the current file installed.
    
    It's practically impossible to rely on version information from
    files (from a quick look, only 5% of the files are versionized and even
    less correctly, libgcrypt seems to do non-monotonic buildid for example)
    
    So the rule that applies when files are not versionized is to check the
    file hash, and the modified date. File hash was added recently in
    msitools, but doesn't apply when the installed file itself has a
    version.
    
    In order to solve the above problems, it's simpler to just have a
    different installation prefix. Windows Installer will see files with
    different component guid, and won't be checking any file update rule.  I
    have verified the upgrade is working, not leaving any file behind and
    updating registry correctly with this solution. Until the files are
    correctly versionized, it looks like the only sensible thing to
    do. Furthermore, this make it simpler to have several versions installed
    in parallel later on (when we change productid)