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. NewProxy 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)