The Mattermost server uses Go modules to manage dependencies.
Adding a dependency is easy. All you have to do is import the dependency in the code and recompile. The dependency will be automatically added for you. Updating uses the same procedure.
Before committing the code with your new dependency added, be sure to run go mod tidy to maintain a consistent format and go mod vendor to synchronize the vendor directory.
If you want to add or update to a specific version of a dependency you can use a command of the form:
go get -u github.com/pkg/errors@v0.8.1
go mod tidy
go mod vendor
If you just want whatever the latest version is, you can leave off the @version tag.
Be sure you have enabled go modules support. After removing all references to the dependency in the code, you run:
go mod tidy
go mod vendor
to remove it from the go.mod file and the vendor directory.