go_embed_data generates different output when it's in an external dependency

If a workspace declaring a go_embed_data target is used as an external dependency for another workspace, the generated Go source's map keys contain external/the_dep_name/ prefixes. This makes it difficult to use from a repo that can be built standalone or as part of a larger project.

Reproduction:

# WORKSPACE
workspace(name = "test_go_embed_data")

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "io_bazel_rules_go",
    urls = ["https://github.com/bazelbuild/rules_go/releases/download/0.18.0/rules_go-0.18.0.tar.gz"],
    sha256 = "301c8b39b0808c49f98895faa6aa8c92cbd605ab5ad4b6a3a652da33a1a2ba2e",
)
load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
go_rules_dependencies()
go_register_toolchains()

local_repository(
    name = "subrepo",
    path = "subrepo",
)

# subrepo/BUILD
load("@io_bazel_rules_go//extras:embed_data.bzl", "go_embed_data")

go_embed_data(
    name = "test_embed",
    srcs = [":BUILD"],
    package = "test_embed",
)

$ bazel build @subrepo//:test_embed
Target @subrepo//:test_embed up-to-date:
  bazel-bin/external/subrepo/darwin_amd64_stripped/test_embed.go

$ grep -A 2 'var Data' bazel-bin/external/subrepo/darwin_amd64_stripped/test_embed.go
var Data = map[string][]byte{
	"external/subrepo/BUILD": Data_0,
}

Start by reproducing the issue with the WORKSPACE and subrepo/BUILD examples, then inspect the go_embed_data entry point loaded from @io_bazel_rules_go//extras:embed_data.bzl and compare standalone and external builds. Done means the generated Data map uses the same key for BUILD in both contexts, without an external/subrepo/ prefix.