Move MIMEType, DetectFormat, ValidateMagicBytes, PeekAndValidate,
IsSupportedMIMEType, MIMEToImageFormat, and ImageFormatToMIME from
internal/imgcache into a new internal/magic package.
The magic package defines its own ImageFormat type and constants to
avoid circular imports (imgcache imports magic for validation,
magic cannot import imgcache).
Update import sites in imgcache/service.go and service_test.go.
Part of issue #39.
