cmd/go/internal/modindex: remove copy of build.Package
modindex defines its own type Package that must be exactly the
same as build.Package (there are conversions between them).
The normal reason to do this is to provide a different method set,
but there aren't any different methods. And if we needed to do that,
we could write
type Package build.Package
instead of repeating the struct definition. Remove the type entirely
in favor of direct use of build.Package.
This makes the modindex package not break when fields are
added to go/build.Package.