feat(gazelle update-repos): Create -strict mode

[tyler-french]

Currently, gazelle update-repos will still operate if someone adds a second duplicate go_repository rule for a repo in the go.mod file. This is part of the confusing part from WORKSPACE dependency management, and may cause confusion when users try to migrate to Bzlmod.

Additionally, it can lead to scenarios where the go_repository rule that is updated by go mod tidy and gazelle update-repos does not match the one used for the build.

Because of these drawbacks, we should create a -strict mode in gazelle update-repos which will fail if duplicate repository rules are found from the repository loader.

The first step in this solution is: #1937