zig build: support dependencies

The `zig build` command now makes `@import("@dependencies")` available
to the build runner package. It contains all the dependencies in a
generated file that looks something like this:

```zig
pub const imports = struct {
    pub const foo = @import("foo");
    pub const @"bar.baz" = @import("bar.baz");
};
pub const build_root = struct {
    pub const foo = "<path>";
    pub const @"bar.baz" = "<path>";
};
```

The build runner exports this import so that `std.build.Builder` can
access it. `std.build.Builder` uses it to implement the new `dependency`
function which can be used like so:

```zig
const libz_dep = b.dependency("libz", .{});
const libmp3lame_dep = b.dependency("libmp3lame", .{});
// ...
lib.linkLibrary(libz_dep.artifact("z"));
lib.linkLibrary(libmp3lame_dep.artifact("mp3lame"));
```

The `dependency` function calls the build.zig file of the dependency as
a child Builder, and then can be ransacked for its build steps via the
`artifact` function.

This commit also renames `dependency.id` to `dependency.name` in the
`build.zig.ini` file.

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