indexer: a big refactor to change how we abort indexing

We replace $dist->{SKIP} with $ctx->dist_errors.  This accumulates a
list of reasons to stop indexing the whole dist (although usually only
one can show up).

$dist->{REASON_TO_SKIP} is also replaced by dist_errors.  Previously,
SKIP (a bool) meant to skip and REASON_TO_SKIP meant "and tell the user
this".  Now, there is always a reason, and the error contains a separate
flag indicating whether to send it to the user.

Previously, some skip reasons were REASON_TO_SKIP, but others were
other properties on the $dist object, like HAS_BLIB or
HAS_MULTIPLE_ROOT.  These are now dist errors, so there's a uniform
handling of their description, and they can abort processing in a
uniform way.

Code that wants to abort can $ctx->abort_indexing, which adds a
dist_error uses an exception to stop processing the dist further.

I deleted a bunch of code managing the "you don't have permissions on
the distribution-name package", which is the thing that asserts you must
have permission on Foo::Bar to upload Foo-Bar.  Previously, this code
still summarized each pmfile scanned, and then fixed up the results to
act like passing modules failed.  Now, this error is simply displayed as
a total dist failure.  I think it should be possible to show the package
indexing summary again in the future, but for now the code was overly
complex and didn't really add much value.

I broke out two hunks of email content generating code, done just to
reduce the total size of the mail_summary subroutine.

Author: rjbs