expose fewer internals in packages, for easier software reuse

- prometheus is now behind an interface, they aren't dependencies for the
  reusable components anymore.
- some dependencies have been inverted: instead of packages importing a main
  package to get configuration, the main package now sets configuration in
  these packages. that means fewer internals are pulled in.
- some functions now have new parameters for values that were retrieved from
  package "mox-".

message/decode.go

@@ -0,0 +1,27 @@
+package message
+
+import (
+	"io"
+	"strings"
+
+	"golang.org/x/text/encoding/ianaindex"
+)
+
+// DecodeReader returns a reader that reads from r, decoding as charset. If
+// charset is empty, us-ascii, utf-8 or unknown, the original reader is
+// returned and no decoding takes place.
+func DecodeReader(charset string, r io.Reader) io.Reader {
+	switch strings.ToLower(charset) {
+	case "", "us-ascii", "utf-8":
+		return r
+	}
+	enc, _ := ianaindex.MIME.Encoding(charset)
+	if enc == nil {
+		enc, _ = ianaindex.IANA.Encoding(charset)
+	}
+	// todo: ianaindex doesn't know all encodings, e.g. gb2312. should we transform them, with which code?
+	if enc == nil {
+		return r
+	}
+	return enc.NewDecoder().Reader(r)
+}