reduce mldistwatch log noise, and use constants (for errors

[rspier]

Primarily

Use a set of constant enums for when mldistwatch decides to ignore a dist.

- The actual values are the same, where possible.  Some values were just '1'
  which has been made more specific.
- These are only used in one place.
- Now we can easily route the log lines for metadata and readme files to the
  debug log

But also a few other cleanups.

[rspier]

Mostly obsoleted by #499 and #501 .