This is potentially an API-breaking change, but it is an important bug fix.

The CL https://golang.org/cl/7305072/ added stuff to make
the tar file look more like a file system internally, including providing an
implementation of os.FileInfo for the file headers within the archive.
But the code is incorrect because FileInfo.Name is supposed to return
the base name only; this implementation returns the full path. A round
trip test added in the same shows this in action, as the slashes are
preserved as we create a header using the local implementation of
FileInfo.

The CL here changes the behavior of the tar (and zip) FileInfo to honor
the Go spec for that interface. It also clarifies that the FileInfoHeader
function, which takes a FileInfo as an argument, will therefore create
a header with only the base name of the file recorded, and that
subsequent adjustment may be necessary.

There may be code out there that depends on the broken behavior.
We can call out the risk in the release notes.

Fixes #6180.

R=golang-dev, dsymonds, adg, bradfitz
CC=golang-dev
https://golang.org/cl/13118043