Fix package structure

[arngrimur]

Today when importingmimetype using go mod vendor some unnecessary files ar e included since they are in the same package as the go files

By moving mime.go, mimetypes.go and tree.go to folder pkg it is possible to reduce the size of vendored files by almost 1.5M. It is not a lot but if applications vendoring lots of libs they accumulate

Thanks!

P:S I can do the work and send a PR if you want