x/tools/cmd/goimports: add -s flag

[cskr]

goimports claims to be a drop-in replacement for editor's gofmt-on-save hook and have
the same command-line interface as gofmt; but it does not support the -s flag.

-s flag can be useful in on-save hook unlike gofmt's -r flag.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-tools, release-none.

[bradfitz]

Comment 2:

It also doesn't support -r.
I think this is a documentation bug. We can say it's "almost a drop-in replacement".

[cskr]

Comment 3:

In which case, can we treat this as a feature request for -s? 
While -r is not very useful in an on-save hook, it'll be nice to have goimports as the
on-save command and include the -s flag.

[bradfitz]

Comment 4:

I ripped it out on purpose, to not duplicate code.
But if Robert is okay with moving it to a package in go.tools so simplify can be shared
by both gofmt and goimports, then I'm okay with it.

[griesemer]

Comment 5:

I am not against it but gofmt (or go fmt) is not depending on anything but the std lib
at the moment.
Note that we are also considering removing that -r option from gofmt; so it may make
sense to remove the -s option as well and have a separate tool instead.
Not for 1.4 for sure.

Status changed to Thinking.