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If 'go install' (with no arguments, meaning the current directory)
succeeds, remove the executable written by 'go build', if present.
This avoids leaving a stale binary behind during a sequence like:

	go build
	<test, mostly works, make small change>
	go install

Before this CL, the current directory still has the stale binary
from 'go build'. If $PATH contains dot, running the name of
the program will find this stale binary instead of the new,
installed one.

Remove the 'go build' target during 'go install', both to clean
up the directory and to avoid accidentally running the stale binary.

Another way to view this CL is that it makes the go command
behave as if 'go install' is implemented by 'go build' followed by
moving the resulting binary to the install location.

See #9645 for discussion and objections.

Fixes #9645.

Change-Id: Ide109572f96bbb5a35be45dda17738317462a7d4
Reviewed-on: https://go-review.googlesource.com/10682

Reviewed-by: Rob Pike <r@golang.org>