Update DEVELOPER.md (#432)
Update DEVELOPER.md (#432)

- Sudo is required for some clang installation commands
- Golang instructions can be simplified
- Other tools need to be installed (`jq`)
- Repository structure if outdated
- Cleanup general guidelines for building and running tests
GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2