Factor out the regular file handling logic into handleRegularFile
from nextHeader. We will need to reuse this logic when fixing #15573
in a future CL.

Factor out the sparse file handling logic into handleSparseFile.
Currently this logic is split between nextHeader (for GNU sparse
files) and Next (for PAX sparse files). Instead, we move this
related code into a single method.

There is no overall logic change. Thus, no unit tests.

Updates #15573 #15564

Change-Id: I3b8270d8b4e080e77d6c0df6a123d677c82cc466
Reviewed-on: https://go-review.googlesource.com/27454Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>