RHEL requires the source code for dependencies to be included in the
srpm. The spec file already expects that, but we've only included the
vendored modules (i.e., the `vendor` directory) in the `rhel-8.2.`
branch. Move vendoring to master, so that we can build RHEL packages
from it as well.
This commit is the result of running `go mod vendor`, which includes the
vendored sources and updates go.mod and go.sum files.
Fedora requires the opposite: dependencies should not be vendored. The
spec file already ignores the `vendor` directory by default.
