Remove support for godep and dep (#1541)

These 2 package managers were archived a while ago
- godep: Feb 26, 2019 (more than 4 years ago)
- dep: Sep 09, 2020 (almost 3 years ago)

Since then, the community has largely adopted Go Module as the
canonicaly package manager.

Removing these old package managers to make it easier to maintain
Gazelle in the long run.
This also let use remove the dependency on pelletier/go-toml package.

(1): https://github.com/golang/dep
(2): https://github.com/tools/godep