Replace various //go:generate lines and regenerate.sh scripts with a
single, top-level regenerate.sh that regenerates all .pb.go files.
Placing generation in a single script ensures that all files are
generated with similar parameters. The new regenerate.sh uses the
protoc-gen-go version defined in test/tools/go.mod and automatically
handles new .proto files as they are added.
Do some minor refactoring on .proto files: Every file now has a
go_package option (which will be required by a future version of the
code generator), and file imports are all relative to the repository
root.
15653fec60