archive/tar: unexport internal methods

Many of the methods inside the archive/tar package don't need to be
exported. Doing so sets a bad precedent that it's OK to export methods
to indicate an internal public API.  That's not a good idea in general,
because exported methods increase cognitive load when reading code:
the reader needs to consider whether the exported method might be used
via some external interface or reflection.

This CL should have no externally visible behaviour changes at all.

Change-Id: I94a63de5e6a28e9ac8a283325217349ebce4f308
Reviewed-on: https://go-review.googlesource.com/c/go/+/341410


Reviewed-by: Joe Tsai <joetsai@digital-static.net>
Trust: Joe Tsai <joetsai@digital-static.net>
Trust: Michael Knyszek <mknyszek@google.com>