gazelle-update-repos should avoid adding duplicates to stack_snapshot packages

[kczulko]

Describe the bug

gazelle-update-repos can generate duplicates within stack_snapshot::packages when one of the packages has "strict" versioning.

To Reproduce

1. Add a "strict versioned" dependency to stack_snapshot::packages list, e.g.

stack_snapshot(
  packages = [
    ...
    "wai-make-assets-0.2", # keep
    ...
  ],
)

2. If gazelle_cabal generated haskell_library contain following entry:

haskell_library(
  deps = [
    ...
    "@stackage//:wai-make-assets",
    ...
  ],
)

...
3. ..., then running :gazelle-update-repos will generate duplicates within stack_snapshot::deps

stack_snapshot(
  packages = [
    ...
    "wai-make-assets",
    "wai-make-assets-0.2", # keep
    ...
  ],
)

It doesn't cause failures for bazel since rules_haskell unify names and allows for a duplicates (but which shouldn't happened as confirmed with @aherrmann - defect is pending and will be added here).

Expected behavior
gazelle-update-repos should try to avoid generating duplicates while updating packages list of the stack repository when one of the packages has #keep comment. Maybe gazelle_cabal should produce additional warning about such case.

Environment

• OS name + version: Nixos 22.05.2351.e4c9d950a3c
• Version of the code:

gazelle_cabal_version = "34a41345973cdffa29bf4d5d349439e86fc30ed3"
rules_haskell_version = "rules_haskell-0.14"

Additional context
Observed while converting example-servant-elm to bazel. There are two places from which those dependencies come:

• hpack's package.yaml: https://github.com/haskell-servant/example-servant-elm/blob/80db4af8d26e39b1e224a0013d1e87007b00393c/package.yaml#L7
• stack's stack.yaml: https://github.com/haskell-servant/example-servant-elm/blob/80db4af8d26e39b1e224a0013d1e87007b00393c/stack.yaml#L9