This doesn't change the existing restriction with disallows
spaces in import paths (as found in an import declaration).
It simply permits packages to be under a directory name that
may contain spaces.

Verified manually that it works. This could use a test, but the
change is trivial. We also can't use the existing test framework
(under test/) because the way those tests are run with test/run.go,
the mechanims for compiling a directory, even if it contains blanks
it its name, does't produce compiler paths with blanks
(the compilation is local).

Fixes #20306.

Change-Id: I6cbffb86c3394347897c3c94b110da0aadc5bfdf
Reviewed-on: https://go-review.googlesource.com/46001Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>