Move tools into separate hack go module

[rifelpet]

This follows a pattern used in other projects moving imports used only in development tools into a separate go module. The new module does not use vendoring. all scripts and make targets that use the tools will now build them into _output/bin/.

I updated the goimports python scripts to rely less on GOPATH but the goimports tool itself still relies on it.

EDIT: i opted to keep go-bindata in the root module because bazel depends on it and I haven't yet figured out how we could best handle that without vendoring go-bindata somewhere:

kops/upup/models/BUILD.bazel

Lines 23 to 30 in c621819

	cmd = """
	$(location //vendor/github.com/go-bindata/go-bindata/go-bindata:go-bindata) \
	-o "$(OUTS)" -pkg models \
	-nometadata \
	-nocompress \
	-prefix $$(pwd) \
	-prefix upup/models $(SRCS)
	""",

also update-machinery.sh still relies on GOPATH, we can update this in a followup PR

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[rifelpet]

I'll need to figure out how to reference the new go-bindata location within this bazel rule

kops/upup/models/BUILD.bazel

Lines 23 to 33 in e789c24

	cmd = """
	$(location //vendor/github.com/go-bindata/go-bindata/go-bindata:go-bindata) \
	-o "$(OUTS)" -pkg models \
	-nometadata \
	-nocompress \
	-prefix $$(pwd) \
	-prefix upup/models $(SRCS)
	""",
	tools = [
	"//vendor/github.com/go-bindata/go-bindata/go-bindata",
	],

[rifelpet]

this is ready for review. as mentioned in the original comment I had to back out some of the moves, we can address them in followup PRs.

[olemarkus]

This looks good! Works as intended locally.

/lgtm

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