Twistedchecker builder should check example code

This is part of the Twisted Better Examples plan:
 * https://twistedmatrix.com/trac/wiki/Plan/BetterExamples#Pyflakesbuildershouldcheckexamplecod

[https://twistedmatrix.com/documents/current/core/development/policy/writing-standard.html#auto8 The writing standard]
suggests that "example code should conform to
the coding standard" which implies that examples should be tested in
the same way as the main twisted code. That includes twistedchecker.

The twistedchecker builder does not currently scan the doc folder and
the Python files within.

 * https://buildbot.twistedmatrix.com/builders/twistedchecker/builds/300/steps/run-twistedchecker/logs/stdio
 * twistedchecker twisted

We should alter the current twistedchecker builder to scan
everything. This would also catch problems with the setuptools files.

{{{
[richard@zorin testable-examples-84]$ find doc -type f -name '*.py' | xargs twistedchecker | wc -l

3150
}}}

But the twistedchecker builder is a "ratchet" test, so these errors
will only show up as "new errors" in initial builds, after which, the
errors can be fixed incrementally in future tickets.

Some of the twistedchecker errors will be due to bugs in
twistedchecker.

Raise tickets for fixing the twistedchecker errors in each
example.

Notes from comments in http://tm.tl/84

We cannot use the find command for this builder.

Instead, twistedchecker should be extended to discover files in
arbitrary directories.