Support custom skylark build flags in external dependencies

[moroten]

Description of the problem / feature request:

The implementation of Skylark support for custom build flags
(#5577) does not work for custom build flags in external repositories.

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Given the following setup:

mkdir my-external
touch my-external/WORKSPACE nothing.txt myext.txt top.txt

cat >my-external/BUILD.bazel <<EOF
load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
bool_flag(
    name = "flag_in_myext",
    build_setting_default = False, visibility = ["//visibility:public"],
)
EOF

cat >BUILD.bazel <<EOF
load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
bool_flag(
    name = "flag_at_top",
    build_setting_default = False,
)
alias(
    name = "alias_for_flag_in_myext",
    actual = "@myext//:flag_in_myext",
)
config_setting(
    name = "is_flag_at_top_enabled",
    flag_values = {":flag_at_top": "True"},
)
config_setting(
    name = "is_flag_in_myext_enabled",
    flag_values = {":alias_for_flag_in_myext": "True"},
)
filegroup(
    name = "everything",
    srcs = select({
        ":is_flag_at_top_enabled": ["top.txt"],
        ":is_flag_in_myext_enabled": ["myext.txt"],
        "//conditions:default": ["nothing.text"],
    }),
)
EOF

cat >WORKSPACE <<EOF
local_repository(name = "myext", path = "my-external")
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "bazel_skylib",
    url = "https://github.com/bazelbuild/bazel-skylib/archive/b113ed5d05ccddee3093bb157b9b02ab963c1c32.zip",
    sha256 = "cea47b31962206b7ebf2088f749243868d5d9305273205bdd8651567d3e027fc",
    strip_prefix = "bazel-skylib-b113ed5d05ccddee3093bb157b9b02ab963c1c32",
)
EOF

the output looks like

$ bazel cquery 'kind("source file", deps(:everything))'
//:nothing.text (null)
$ bazel cquery 'kind("source file", deps(:everything))' --//:flag_at_top
//:top.txt (null)
$ bazel cquery 'kind("source file", deps(:everything))' --@myext//:flag_in_myext
ERROR: Unrecognized option: --@myext//:flag_in_myext
$ bazel cquery 'kind("source file", deps(:everything))' --//:alias_for_flag_in_myext
ERROR: Unrecognized option: //:alias_for_flag_in_myext

I would expect the bottom two cases to work and output //:myext.txt (null). The alias would be very convenient for the user to shorten the command line.

What operating system are you running Bazel on?

Linux

What's the output of bazel info release?

release 1.0.0

Have you found anything relevant by searching the web?

@gregestren suggested reporting this bug/feature request as the core functionality was implemented in #5577.

[gregestren]

This is worth at least clarifying which part of the code base is failing to make the connection. From there we can prioritize the right fix.

[jmillikin-stripe]

PR to support Starlark-defined flags starting with --@: #10052