Allows reading -importmap options from a file instead of putting
them all on the command line, and adds the ability to specify the
file location of specific packages. In effect, -importcfg is a generalization
of and supersedes -importmap, -importsuffix, and -I.
Of course, those flags will continue to be supported,
for compatibility with other tools.

Having this flag in Go 1.9 will let us try some experiments involving
package management without needing guinea pigs to build a
custom Go toolchain.

This flag also helps with #14271 at some later point.

For #20579.

Change-Id: If005dbc2b01d8fd16cbfd3687dfbe82499f4bc56
Reviewed-on: https://go-review.googlesource.com/44850
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>