Standardize repository rule naming for 3rd party packages.

I recently observe in https://github.com/aherrmann/rules_zig/ that the toolchain is managed via a rule called `zig_repository`, which follow the template convention of `mylang_repository`.

However, what I am used to in Go is `go_repository` is used to manage 3rd party dependency import. Toolchain in Go is managed via `go_{download,local,host}_sdk` and `go_register_toolchains` rules instead.

So I would suggest renaming `mylang_repository` to `mylang_sdk` and `mylang_register_toolchains` while reserving `mylang_repository` for respective language's 3rd party import. WDYT?

Start by comparing the `rules_zig` naming example and the Go rule conventions cited in the issue with the template's generated rule names. Determine whether `mylang_repository` and the toolchain rules should be renamed or reserved differently. Done means the naming decision is agreed and any affected template references are updated consistently.

Written by the indexing model from the issue text.