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

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

Following discussion on #43724, change os/exec to take the
approach of golang.org/x/sys/execabs, refusing to respect
path entries mentioning relative paths by default.

Code that insists on being able to find executables in relative
directories in the path will need to add a couple lines to override the error.

See the updated package docs in exec.go for more details.

Fixes #43724.
Fixes #43947.

Change-Id: I73c1214f322b60b4167a23e956e933d50470fe13
Reviewed-on: https://go-review.googlesource.com/c/go/+/381374
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>