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. They reach
the cache through its database, its state directory and its metrics registry,
and run its reconciliation pass with a second cache, as pixad does at startup.

Model: opus-5-5

internal/imgcache/eviction.go

@@ -317,7 +317,7 @@ func (c *Cache) evictVariant(
 // evictSourceBlob removes one source: its row first, then its file, as
 // evictVariant does for a variant. It holds no lock: a store of the same
 // source in between can leave the source's row with no file, which
-// LookupSource treats as a miss.
+// Source treats as a miss.
 func (c *Cache) evictSourceBlob(ctx context.Context, key imagerequest.SourceKey) error {
 	_, err := c.db.ExecContext(ctx,
 		`DELETE FROM source_content WHERE source_key = ?`, string(key))
@@ -479,7 +479,7 @@ func (c *Cache) runCleanExpired(ctx context.Context) {
 // then on the first periodic eviction tick that comes at least ten times
 // as long after the last pass ended as that pass took, off the request
 // hot path: it adopts variant files that predate the accounting table
-// (or whose accounting insert failed, e.g. StoreVariant's best-effort
+// (or whose accounting insert failed, e.g. PutVariant's best-effort
 // insert under transient DB contention), drops variant accounting rows
 // whose files are missing, removes source blob files the database does
 // not know, removes source and variant files that are empty or not the