x/build: unify list, configuration of repos, simplifying add new ones

[bradfitz]

https://github.com/golang/go/wiki/CreatingSubRepository is super complicated.

We had our list of Go repos is a half dozen places.

https://golang.org/cl/208697 added a new repos package for use in x/build and uses it in a number of places.

https://golang.org/cl/210461 fleshes it out more and uses it for maintnerd, removing another list.

Remaining:

• gerritbot.go's

// Gerrit projects we accept PRs for.                                                                                                                                                                      
var gerritProjectWhitelist = map[string]bool{
        "arch":           true,
        "benchmarks":     true,
        "blog":           true,
        "build":          true,
        "crypto":         true,
        "debug":          true,
        "dl":             true,
        "example":        true,
...

• simplify the CreatingSubRepository wiki page.

[gopherbot]

Change https://golang.org/cl/210461 mentions this issue: repos: flesh out package more, use it in maintnerd

[gopherbot]

Change https://golang.org/cl/210741 mentions this issue: cmd/gerrit: use the new repos package for repo policy

[gopherbot]

Change https://golang.org/cl/210744 mentions this issue: cmd/golangorg: remove duplicate list of repo names from the /x/ handler

[gopherbot]

Change https://golang.org/cl/211318 mentions this issue: repos: add descriptions from golang.org/pkg/#subrepo

[bradfitz]

Updated the wiki. I think this is done. If people find more lists of repos we can reopen this.

[dmitshur]

If people find more lists

I found two more, but I don't think they're worth implementing right now, because it's a lot of forced changes and not a lot of benefit. I've opened issue #36136 to track them so that it can be considered after more progress is made on #29206 and it becomes a lot easier to implement.