Replace libdparse with DMD in HasPublicExampleCheck

[Vladiwostok]

This check warns for public and documented declarations that do not have a documented unit test attached. This check is fairly modest, as a documented unit test "attaches" only to the previous declaration (so there's a possibility that there's a unit test that doesn't test a declaration, and it won't warn).
In libdparse version, I've noticed a few things:

• the module is not visited, however it's declarations are directly checked; this was not identically possible in DMD;
• old check only looks for a few declarations: classes, interfaces, structs, unions, functions and templates;
• because of the check's behavior and libdparse Module ASTNode, conditional declarations are completely ignored (such as a module-level 'version' block) - I've kept the same behavior in DMD version;
• the check's DDoc mentions that enums are not checked, however enum checking does appear in code; on further inspections (and testing it against enums in phobos), I've noticed that enums are in fact NOT checked, due to libdparse Module ASTNode; I've kept the same behaviour;
• the visitor intends to skip non-public declarations, however in the actual code only non-private, non-protected or non-package declarations are checked (that means 'export' declarations are also checked) - kept the same behavior, but should this be intended?

[RazvanN7]

Mind the conflict.