extract WriteSourceFileStep out from WriteFileStep

[andrewrk]

Extracted from #14647.

zig/lib/std/Build/WriteFileStep.zig

Lines 5 to 10 in b3af5d0

	//! However, this step has an additional capability of writing data to paths
	//! relative to the package root, effectively mutating the package's source
	//! files. Be careful with the latter functionality; it should not be used
	//! during the normal build process, but as a utility run by a developer with
	//! intention to update source files, which will then be committed to version
	//! control.

zig/lib/std/Build/WriteFileStep.zig

Lines 148 to 186 in b3af5d0

	// Writing to source files is kind of an extra capability of this
	// WriteFileStep - arguably it should be a different step. But anyway here
	// it is, it happens unconditionally and does not interact with the other
	// files here.
	var any_miss = false;
	for (wf.output_source_files.items) |output_source_file| {
	if (fs.path.dirname(output_source_file.sub_path)) |dirname| {
	b.build_root.handle.makePath(dirname) catch |err| {
	return step.fail("unable to make path '{}{s}': {s}", .{
	b.build_root, dirname, @errorName(err),
	});
	};
	}
	switch (output_source_file.contents) {
	.bytes => |bytes| {
	b.build_root.handle.writeFile(output_source_file.sub_path, bytes) catch |err| {
	return step.fail("unable to write file '{}{s}': {s}", .{
	b.build_root, output_source_file.sub_path, @errorName(err),
	});
	};
	any_miss = true;
	},
	.copy => |file_source| {
	const source_path = file_source.getPath(b);
	const prev_status = fs.Dir.updateFile(
	fs.cwd(),
	source_path,
	b.build_root.handle,
	output_source_file.sub_path,
	.{},
	) catch |err| {
	return step.fail("unable to update file from '{s}' to '{}{s}': {s}", .{
	source_path, b.build_root, output_source_file.sub_path, @errorName(err),
	});
	};
	any_miss = any_miss or prev_status == .stale;
	},
	}
	}

This should be a different step, not part of WriteFileStep.