add resolveTargets function for improved lazy import support

[silbinarywolf]

Based on request here: #82

Example usage:

// Example usage in Zig 0.16.0-dev.3061+9b1eaad13
pub fn build(b: *std.Build) void {
    const exe_name: []const u8 = "minimal";
    const root_target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const all_android_targets = b.option(bool, "android", "Build for all Android targets (x86, x86_64, aarch64, arm, etc)") orelse false;
    const android_targets: []std.Build.ResolvedTarget = blk: {
        if (all_android_targets or root_target.result.abi.isAndroid()) {
            if (b.lazyImport(@This(), "android")) |android| {
                break :blk android.resolveTargets(b, .{
                    .default_target = root_target,
                    .all_targets = all_android_targets,
                });
            }
        }
        break :blk &[0]std.Build.ResolvedTarget{};
    };

I opted to look at how Zig separates build option creation for targets and took a similar route, for example Zig has resolveTarget for handling the logic without invoking b.option

/// Exposes standard `zig build` options for choosing a target and additionally
/// resolves the target query.
pub fn standardTargetOptions(b: *Build, args: StandardTargetOptionsArgs) ResolvedTarget {
    const query = b.standardTargetOptionsQueryOnly(args);
    return b.resolveTargetQuery(query);
}