Move HTTPFetcher, Config (was FetcherConfig), SSRF-safe dialer, rate
limiting, content-type validation, and related error vars from
internal/imgcache/fetcher.go into new internal/httpfetcher/ package.
The Fetcher interface and FetchResult type also move to httpfetcher
to avoid circular imports (imgcache imports httpfetcher, not the other
way around).
Renames to avoid stuttering:
NewHTTPFetcher -> httpfetcher.New
FetcherConfig -> httpfetcher.Config
NewMockFetcher -> httpfetcher.NewMock
The ServiceConfig.FetcherConfig field is retained (it describes what
kind of config it holds, not a stutter).
Pure refactor - no behavior changes. Unit tests for the httpfetcher
package are included.
refs #39
