tech debug: migrate tooling to take advantage of Go 1.16

[alexec]

A lot of our build is designed to deal with shortcomings of Go 1.15 which are fixed in Go 1.16:

• Install tools using go install instead of go get to pin the versions:

go get golang.org/x/tools/cmd/goimports
# becomes 
go install golang.org/x/tools/cmd/goimports@v0.1.7 

• Get rid of tools.go.
• Replace staticfiles with Golang 1.16 built-in feature.

This is the first step to Dockerize tooling as per #6519 and #6618, #6890 .

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[alexec]

Replace staticfiles with Golang 1.16 built-in feature.

I think we should leave this. It uses the same patterns as plenty of other tools we use.

[agilgur5]

Replace staticfiles with Golang 1.16 built-in feature.

I think we should leave this. It uses the same patterns as plenty of other tools we use.

For posterity, we are moving off of staticfiles to the embed built-in per #11654 (+ #11052). CD uses it nowadays and staticfiles is also archived nowadays (see the issue for more details).