(fix) emitCss: use deterministic css file names

[cristinecula]

I have read over the thread you mentioned on my previous pull request (webpack/watchpack#25) and It struck me that there is another issue with the current implementation:

Normally multiple rebuilds shouldn't cause issues and this cause should be cached by the caller by comparing hash. I.e. the webpack CLI doesn't display stats output for equal hash.

If you run webpack multiple times over the same Svelte file, the output hash is different each time. If you take out the <style> tag, then the hash remains the same. This happens because a new temporary css file with a random name is generated each time. This could prove annoying to developers using code splitting or relying on output files including [hash] (https://medium.com/react-weekly/code-chunking-with-webpack-a-pragmatic-approach-e17e8bcc6453).

I think a safe approach would be to use ast.hash as part of the temporary file name:

	if (options.emitCss && css) {
		const tmpFile = path.join(os.tmpdir(), 'svelte-' + ast.hash + '.css');

		css += '\n/*# sourceMappingURL=' + cssMap.toUrl() + '*/';
		code = code + `\nrequire('${tmpFile}');\n`;

		writeFileSync(tmpFile, css);
		const stats = statSync(tmpFile);
		utimesSync(tmpFile, stats.atimeMs - 99999, stats.mtimeMs - 99999);
	}

The downside is that the tmp package cannot be used to generate files without the random bits (it uses mkstmp under the hood) and thus it should be removed, but it also did a cleanup of the temp files when the process exited. Without this, the /tmp folder is cleaned only after a system reboot. I'm not sure how important this is.

[Rich-Harris]

Thanks! Released this as 2.2.2.

Without this, the /tmp folder is cleaned only after a system reboot. I'm not sure how important this is.

I think it's fine — as long as they eventually get cleaned up, I don't see it as a big deal at all.