One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's short CBOR field names. VariantKey moves with the function that
makes it. Tests follow the moved and renamed code; the tests of the
removed conversions go with them.

Model: opus-5-5

internal/imgcache/cache_usage_internal_test.go

@@ -5,6 +5,8 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestEvictionReadsTheUsageTotal adds, stores again, resizes and evicts
@@ -171,7 +173,8 @@ func TestReconciliationReadsAPageAtATime(t *testing.T) {
 
 		storeEvictionTestSource(t, cache, "pages.example.com",
 			"/"+strconv.Itoa(i)+".jpg", bytes.Repeat(content, 100))
-		storeEvictionTestVariant(t, cache, VariantKey("aabbccdd000"+strconv.Itoa(i)),
+		storeEvictionTestVariant(t, cache,
+			imagerequest.VariantKey("aabbccdd000"+strconv.Itoa(i)),
 			bytes.Repeat(content, 10))
 	}