os/exec: return error when PATH lookup would use current directory

CL 381374 was reverted because x/sys/execabs broke.

This CL reapplies CL 381374, but adding a lookPathErr error
field back, for execabs to manipulate with reflect.
That field will just be a bit of scar tissue in this package forever,
to keep old code working with new toolchains.

CL 403256 fixes x/sys/execabs's test to be ready for the change.
Older versions of x/sys/execabs will keep working
(that is, will keep rejecting what they should reject),
but they will return a slightly different error from LookPath
without that CL, and the test fails because of the different
error text.

For #43724.

This reverts commit f2b674756b3b684118e4245627d4ed8c07e518e7.

Change-Id: Iee55f8cd9939e1bd31e5cbdada50681cdc505117
Reviewed-on: https://go-review.googlesource.com/c/go/+/403274
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

0 files changed, 0 insertions, 0 deletions