Name the source of each config option in diagnose

[unflxw]

The diagnose report prints every configuration option with the value it
holds, and nothing else. An option that is not what the application set
is the first thing somebody looks at, and the report gives them no way
to tell a default apart from a value read from the environment, so the
next step is guesswork. The Ruby gem's report has named the source for
years.

Print the source beside the value, and, for an option more than one
source holds, list each source with what it holds in the order they are
merged. The order comes from the same constant the merge walks, so the
report cannot disagree with the merge about which value wins.

The diagnose test expectations move with it, in diagnose_tests#159.

Kept apart from #287 so the two can be judged on their own. When both land, derived joins SOURCE_ORDER and the expectations gain a line for it.

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[unflxw]

Folded into #287. The two changes need each other: #287 adds the derived configuration source, and this is what makes it visible in the diagnose report. Kept apart they would each ship a report that does not name one of the sources the package merges.