In the near future, the subchecks of 'fields' will stand on their own.
They will no longer be invoked from inside the old check.

This associates the tags emitted by each subcheck with that subcheck.

The split was easy except for maintainer and uploaders, which are
somewhat awkwardly issued by a library module. (There are many
similarities in the way they are processed.) A better way would be for
the library to evaluate the fields, but for the checks to issue their
own tags.

Created '*.desc' files for the subchecks so that Lintian recognizes
them as checks that stand on their own.

These checks were previously invoked via the main check. The main
check will be removed in the near future.

The constituent checks now stand on their own. The old module that
used to call them is no longer needed. It is removed here.

Gbp-Dch: ignore

Removes the old check 'fields' from the profile debian/main. Adds the
subcheck, which now stand on their own, to the profile instead.

Tests were copied or duplicated as needed whenever it helped to
evaluate a subcheck. The two scenarios were either that the expected
tags or the tags listed in Test-Against related to the subcheck.

One exception was the test fields-depends-wildcard, which had neither
expected any tags nor defined a Test-Against. Upon examination, that
test was added to the check 'fields/package-relations' with
appropriate settings in Test-Against.

Splits all tests for the old check 'fields' into the subtests so they
can stand on their own.

Gbp-Dch: ignore

In Lintian's output, tags appear in the order they are emitted.
Adjusts literal tests for changes is check execution order.

This commit contains no substantive changes.

Gbp-Dch: ignore