spec: Load an entire `.bzl` file as a single symbol (glob/star imports)

[jmillikin]

It is relatively common to see a struct used to aggregate a module's public API, for example in https://github.com/bazelbuild/rules_cc/blob/0.2.17/cc/common/cc_helper.bzl

cc_helper = struct(
    rule_error = _rule_error,
    attribute_error = _attribute_error,
    # ... a lot of symbols ...
    package_exec_path = _package_exec_path,
    should_create_test_dwp_for_statically_linked_test = _should_create_test_dwp_for_statically_linked_test,
)

This pattern is used to simplify the load for that module:

load("@rules_cc//cc/common:cc_helper.bzl", "cc_helper")

There's been some proposals to add glob/star imports in the past (example: #111) but they haven't been adopted due to breaking syntax compatibility.

I propose to add this syntax:

# load the entire `helpers.bzl` namespace under the `helpers` symbol
load("@example//path/to/helpers.bzl", helpers = "*")

# roughly equivalent to:
load("@example//path/to/helpers.bzl", _sym0 = "do_something", _sym1 = "do_something_else")
helpers = struct(
    do_something = _sym0,
    do_something_else = _sym1,
)

This would enable new code to have simpler imports while maintaining syntax compatibility with existing AST parsers and formatters.

[darkuranium]

For my $0.02, I can't find the issue right now, but one of my own ideas was this (which does extend the syntax, but only in a simple and Python-compatible manner:

helpers = load("@example//path/to/helpers.bzl")

It's not 100% backwards syntax-compatible (but then again, nor is helpers = "*" semantically-compatible, and that's what matters at the end of the day), but I really like the ergonomics; the syntax feels nice & obvious, and not unlike helpers = require "..." in Lua or helpers = require("...") in (old/CommonJS) JS.