v0.4.15: file.imports() pulls in all transitive dependencies (#76)

this PR fixes two bugs related to file.imports() and simplifies pulling in a file's dependencies

- file.imports() will now pull in all transitive dependencies, rather than just immediate dependencies
- file.imports() correctly pulls in all immediate dependencies via file.descriptor.GetDependency() removing the need to check a file's messages and services for additional imports
- updates go vet command and go version https://golang.org/doc/go1.12#vet