cmd/weave: move weave to a common location

Move the weave program out of the gotypes directory
so it can be used by other example documents.

It will soon be used for a document on writing slog handlers.

Change-Id: I38255e276e744e323695afaa66830107aa17d4d1
Reviewed-on: https://go-review.googlesource.com/c/example/+/494441
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: jba

gotypes/Makefile

@@ -2,8 +2,8 @@
 all: README.md
 	go build ./...
 
-README.md: go-types.md weave.go $(wildcard */*.go)
-	go run weave.go go-types.md >README.md
+README.md: go-types.md ../internal/cmd/weave/*.go $(wildcard */*.go)
+	go run ../internal/cmd/weave go-types.md >README.md
 
 # This is for previewing using github.
 # $HOME/markdown must be a github client.