switch to slog.Logger for logging, for easier reuse of packages by external software

we don't want external software to include internal details like mlog.
slog.Logger is/will be the standard.

we still have mlog for its helper functions, and its handler that logs in
concise logfmt used by mox.

packages that are not meant for reuse still pass around mlog.Log for
convenience.

we use golang.org/x/exp/slog because we also support the previous Go toolchain
version. with the next Go release, we'll switch to the builtin slog.

develop.txt

@@ -10,6 +10,14 @@ This file has notes useful for mox developers.
   name/path manipulation. Do not remove/rename files that are still open.
 - Not all code uses adns, the DNSSEC-aware resolver. Such as code that makes
   http requests, like mtasts and autotls/autocert.
+- We don't have an internal/ directory, really just to prevent long paths in
+  the repo, and to keep all Go code matching *.go */*.go (without matching
+  vendor/). Part of the packages are reusable by other software. Those reusable
+  packages must not cause mox implementation details (such as bstore) to get out,
+  which would cause unexpected dependencies. Those packages also only expose the
+  standard slog package for logging, not our mlog package. Packages not intended
+  for reuse do use mlog as it is more convenient. Internally, we always use
+  mlog.Log to do the logging, wrapping an slog.Logger.
 
 
 # TLS certificates