all: use os/exec instead of internal/execabs

all: use os/exec instead of internal/execabs

We added internal/execabs back in January 2021 in order to fix
a security problem caused by os/exec's handling of the current
directory. Now that os/exec has that code, internal/execabs is
superfluous and can be deleted.

This commit rewrites all the imports back to os/exec and
deletes internal/execabs.

For #43724.

Change-Id: Ib9736baf978be2afd42a1225e2ab3fd5d33d19df
Reviewed-on: https://go-review.googlesource.com/c/go/+/381375
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>