Split internal/imgcache into the image proxy and the cache (closes #317)

The new package internal/imageproxy holds Proxy, which was imgcache.Service,
with its Config, ImageResponse, CacheStatus and errors. imageproxy.New makes
and registers the counters of upstream fetches and transforms.
internal/imgcache holds the cache only, with its hit and miss counters, and
imgcache.CacheConfig is imgcache.Config. The cache reads and writes each kind
it keeps with one pair of methods: Variant and PutVariant, Source and
PutSource, Failure and PutFailure. Source replaces LookupSource followed by
GetSourceContent. imgcache.go and module.go are gone. The proxy's tests moved
with it. The cache's tests check that the reconciliation pass removes an empty
or cut file and its row. The proxy's tests remove them by hand, then check
that Get fetches or converts again.

Model: opus-5-5

internal/imgcache/counters.go

@@ -19,22 +19,8 @@ func (c *Cache) registerCounters(metricsRegistry prometheus.Registerer) error {
 		Name: "pixa_cache_misses_total",
 		Help: "Image requests not served from the cache.",
 	})
-	c.upstreamFetches = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "pixa_upstream_fetches_total",
-		Help: "Fetches of a source image from upstream that read any bytes.",
-	})
-	c.upstreamFetchBytes = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "pixa_upstream_fetch_bytes_total",
-		Help: "Bytes read from upstream by fetches of source images.",
-	})
-	c.transforms = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "pixa_transforms_total",
-		Help: "Images the image processor transformed.",
-	})
 
-	for _, counter := range []prometheus.Counter{
-		c.cacheHits, c.cacheMisses, c.upstreamFetches, c.upstreamFetchBytes, c.transforms,
-	} {
+	for _, counter := range []prometheus.Counter{c.cacheHits, c.cacheMisses} {
 		err := metricsRegistry.Register(counter)
 		if err != nil {
 			return fmt.Errorf("failed to register the cache's counters: %w", err)