Expose less awkward `Files` type in `Report` struct

[egibs]

We should store Files as a map[string]*FileReport type rather than sync.Map in the Report struct. We can use an intermediary struct for the concurrent operations and then build the final report with the more friendly type.

I already have working code for this and should have the PR up soon.

[egibs]

It probably makes sense to land this after the #734 refactor since that will change the layout of scan.go.