compiler: Record each import as a distinct alias.

    
    This patch introduces the Package_alias class which is a
    finer representation of the different between a package and the
    aliases it is imported under.  Each alias keeps track of the location
    of its import statement and how many times that alias has been used.
    This allows the gofrontend to report when a specific import has not
    been used even if a symbol from the package has been used by another
    import.
    
    Fixes golang/go#12326.
    
    Reviewed-on: https://go-review.googlesource.com/14259

From-SVN: r228550

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