go_library: depending on multiple packages with the same import path should be an error

[jayconrod]

What version of rules_go are you using?

v0.23.0

What version of gazelle are you using?

n/a

What version of Bazel are you using?

3.1.0

Does this issue reproduce with the latest releases of all the above?

yes

What operating system and processor architecture are you using?

Darwin / amd64

Any other potentially useful information about your toolchain?

n/a

What did you do?

bazel build //:a
-- WORKSPACE --
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "io_bazel_rules_go",
    sha256 = "6a68e269802911fa419abb940c850734086869d7fe9bc8e12aaf60a09641c818",
    urls = [
        "https://mirror.bazel.build/github.com/bazelbuild/rules_go/releases/download/v0.23.0/rules_go-v0.23.0.tar.gz",
        "https://github.com/bazelbuild/rules_go/releases/download/v0.23.0/rules_go-v0.23.0.tar.gz",
    ],
)

load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")

go_rules_dependencies()

go_register_toolchains()
-- BUILD.bazel --
load("@io_bazel_rules_go//go:def.bzl", "go_library")

go_library(
    name = "a",
    importpath = "a",
    deps = [
        ":b1",
        ":b2",
    ],
)

go_library(
    name = "b1",
    importpath = "b",
    importmap = "b1",
)

go_library(
    name = "b2",
    importpath = "b",
    importmap = "b2",
)

What did you expect to see?

The build should fail because a depends on two targets with importpath = "b". They can't both be imported.

What did you see instead?

Successful compilation.

[jayconrod]

Removing from v0.24 milestone. This appears to be supported in _dedup_deps called from go.library_to_source. Later dependencies are ignored. That allows an embedding library to override the dependencies of embedded libraries.

I'm not sure if that's a good idea, but fixing this may be an incompatible change, so setting milestone to v1.0.0.