Use cache='reload' for precaching unversioned URLs

[jeffposnick]

I'm spinning this off of #2143 (comment)

In Workbox v3, we used code like

workbox/packages/workbox-precaching/models/PrecacheEntry.mjs

Lines 54 to 77 in d27aafb

	_cacheBustRequest(request) {
	let url = request.url;
	const requestOptions = {
	credentials: 'same-origin',
	};
	if ('cache' in Request.prototype) {
	// Make use of the Request cache mode where we can.
	// Reload skips the HTTP cache for outgoing requests and updates
	// the cache with the returned response.
	requestOptions.cache = 'reload';
	} else {
	const parsedURL = new URL(url, location);
	// This is done so the minifier can mangle 'global.encodeURIComponent'
	const _encodeURIComponent = encodeURIComponent;
	parsedURL.search += (parsedURL.search ? '&' : '') +
	_encodeURIComponent(`_workbox-cache-bust`) + '=' +
	_encodeURIComponent(this._revision);
	url = parsedURL.toString();
	}
	return new Request(url, requestOptions);
	}

to cache-bust an outgoing request to populate the precache via setting the cache property to 'reload', with some feature detection to fall back to setting a cache-bust query parameter if needed.

In v4, we stopped that (partly because we end up using that query parameter as the actual cache key, so it's somewhat simpler to also use it in the outgoing request.)

We can change that, though, and potentially just set cache = 'reload' again. I did a quick check and it's supported in all modern browsers; you can check with:

fetch('https://httpbin.org/headers', {cache: 'reload'}).then(r => r.text()).then(console.log);

This might be done as part of a refactoring that would clear up some of our naming in workbox-precache around cacheKey and url.

CC: @philipwalton