io: unexport internal methods

The methods on the pipe type 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.

Change-Id: Ib13f1b3f9fe0ff251628f31b776182a0953268ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/341409


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Joe Tsai <joetsai@digital-static.net>
Trust: Daniel Martí <mvdan@mvdan.cc>