cmd/go: use os.Executable to find GOROOT

cmd/go: use os.Executable to find GOROOT

Before this change, building a GOROOT using make.bash, and then
moving the entire to a new path confused the go tool. Correct
operation of the go tool under these conditions required either
running make.bash again (not always possible if the new location
was owned by a different system user) or setting the GOROOT
environment variable. Setting GOROOT is unfortunate and
discouraged, as it makes it too easy to use the go tool from
one GOROOT and the compiler from another GOROOT.

With this change, the go tool finds its GOROOT relative to its
own location, using os.Executable. It checks it is in a GOROOT
by searching for the GOROOT/pkg/tool directory, to avoid two
plausible situations:

	ln -s $GOROOT/bin/go /usr/local/bin/go

and

	PATH=$HOME/bin:$PATH
	GOPATH=$HOME
	ln -s $GOROOT/bin/go $HOME/bin/go

Additionally, if the current executable path is not in a GOROOT,
the tool will follow any symlinks for the executable and check
to see if its original path is a GOROOT.

Fixes #18678

Change-Id: I151d7d449d213164f98193cc176b616849e6332c
Reviewed-on: https://go-review.googlesource.com/42533
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>