break up TD004 into multiple plugins

[DinoChiesa]

TD004 does a bunch of hygiene checks for HTTPTargetConnection. We need to separate out some of the function into new, separate plugins.

Now that we have the <!-- apigeelint disable XXXXX --> directive, people might want to disable some of the checks currently done by TD004, like the SSLInfo related things, but not others, like duplicate URL elements. Separating these checks into smaller related bunches , each contained in a new plugin, will allow better control.

Refactoring TD004 may cause a back-compatibility issue for anyone who is currently disabling TD004. After we introduce one or more additional plugins, they'll get warnings again. This should be a minor issue though, comparable to any case in which we introduce a new builtin plugin.

[DinoChiesa]

addressed in #478