Now that we have -importcfg, there's no need for the
temporary directory trees that mirror the import path structure,
and we can drop a bunch of complex code that was building
and maintaining that structure.

This should fix "file name too long" errors on systems with low limits.
(For example #10651 and #17070, although we fixed those by
adding code to deal with very long file names on Windows instead.)

Change-Id: I11e221c6c1edeb81c3b2f1d89988f5235aa2bbb9
Reviewed-on: https://go-review.googlesource.com/56280Reviewed-by: Ian Lance Taylor <iant@golang.org>