make `@embedFile` support module-mapped names the same way as `@import`

[andrewrk]

Concrete example:

bootloader.zig

pub export fn _start() void {}

main.zig

const std = @import("std");
const blah = @embedFile("bootloader.elf");

pub fn main() !void {
    std.debug.print("first 3 bytes: " ++ blah[0..3] ++ "\n", .{});
}

build.zig

const std = @import("std");

pub fn build(b: *std.Build) void {
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const bootloader = b.addExecutable(.{
        .name = "bootloader",
        .root_source_file = .{ .path = "bootloader.zig" },
        .target = .{
            .cpu_arch = .x86,
            .os_tag = .freestanding,
        },
        .optimize = .ReleaseSmall,
    });

    const exe = b.addExecutable(.{
        .name = "embed-generated-file",
        .root_source_file = .{ .path = "main.zig" },
        .target = target,
        .optimize = optimize,
    });
    exe.addAnonymousModule("bootloader.elf", .{
        .source_file = bootloader.getOutputSource(),
    });
    exe.install();
}

---

This fully works according to the build system, but compilation of main.zig fails like this:

andy@ark ~/m/embed-generated-file> zig build
main.zig:2:25: error: unable to open 'bootloader.elf': FileNotFound
const blah = @embedFile("bootloader.elf");
                        ^~~~~~~~~~~~~~~~
referenced by:
    main: main.zig:5:42
    callMain: /home/andy/Downloads/zig/lib/std/start.zig:616:32
    remaining reference traces hidden; use '-freference-trace' to see all reference traces

error: embed-generated-file...
error: The following command exited with error code 1:
/home/andy/Downloads/zig/build-release/stage3/bin/zig build-exe /home/andy/misc/embed-generated-file/main.zig --cache-dir /home/andy/misc/embed-generated-file/zig-cache --global-cache-dir /home/andy/.cache/zig --name embed-generated-file --pkg-begin bootloader.elf /home/andy/misc/embed-generated-file/zig-cache/o/4fbab5d6b4239cb2e694a118ffff2202/bootloader --pkg-end --enable-cache 

You can see that it did, indeed, pass these flags:

--pkg-begin
bootloader.elf
/home/andy/misc/embed-generated-file/zig-cache/o/4fbab5d6b4239cb2e694a118ffff2202/bootloader
--pkg-end

This issue is to make zig honor those flags, and thereby allow @embedFile("bootloader.elf") to work.

[michelepagot]

For whom land on this suggestion later, addAnonymousModule has been removed in #18160