rules_haskel allows for "duplicates" within stack_snapshot::packages

[kczulko]

Describe the bug
Accidentally, I've discovered that stack_snapshot packages list may contain "quasi-duplicates".

To Reproduce
Put two packages to stack_snapshot::packages where one is taken from stack snapshot and the other one contains a strict version, e.g:

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

Expected behavior
While, in duplicated such case, using reference without the version is allowed for targets of type e.g. haskell_library:

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

rules_haskell should probably generate an error. For a reader it may be a bit misleading what's happening in such case. Does the dependency come from stackage or hackage (https://rules-haskell.readthedocs.io/en/latest/haskell-use-cases.html#version-overrides-or-hackage-dependencies)?

Environment

• OS name + version: Nixos 22.05.2351.e4c9d950a3c
• Bazel version: bazel 4.1.0-
• Version of the rules: "rules_haskell-0.14"

Additional context
tweag/gazelle_cabal#46

[aherrmann]

Closed in #1815