internal/fuzz: OSS-Fuzz build is failing

[neild]

See:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32233

Failure looks real:

Step #3: + . internal/fuzz/oss-fuzz-build.sh
Step #3: ++ for x in 'internal/fuzz/*'
Step #3: ++ '[' -d internal/fuzz/README.md/corpus ']'
Step #3: ++ for x in 'internal/fuzz/*'
Step #3: ++ '[' -d internal/fuzz/jsonfuzz/corpus ']'
Step #3: +++ basename internal/fuzz/jsonfuzz
Step #3: ++ name=jsonfuzz
Step #3: ++ compile_go_fuzzer google.golang.org/protobuf/internal/fuzz/jsonfuzz Fuzz jsonfuzz protolegacy
Step #3: go: downloading github.com/google/go-cmp v0.5.5
Step #3: go: downloading github.com/golang/protobuf v1.5.0
Step #3: go: downloading golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543
Step #3: google.golang.org/protobuf/internal/fuzz/jsonfuzz
Step #3: google.golang.org/protobuf/internal/fuzz/jsonfuzz
Step #3: go list -m: module google.golang.org/protobuf/internal: not a known dependency
Step #3: no required module provides package google.golang.org/protobuf/internal; to add it:
Step #3: 	go get google.golang.org/protobuf/internal

Not sure if this is a problem with the OSS-Fuzz-provided compile_go_fuzzer function or our own build script.

[dsnet]

I've been using the dev.fuzz branch of the Go toolchain and it's quite a bit easier to use since it's natively integrated into the Go toolchain itself. Given that golang/go#44551 is accepted, I might wait for that unless one of us get around to investigating and fixing this first.