Yang code generator cleanup

Yang code generator cleanup

Two axes:
* make the generated code slightly better
* make the generator code slightly better

Fixes in generator code:
* fields which can be local variables
* map overwrite detection using put()'s return value
* logging with exceptions
* using File's parent/child handling instead of concatenating strings
* if (condition) { return true; } else { return false; }
  simplification
* if (... == false) -> if (!...)
* interface method declarations are public by default
* Collections.singletonList() instead of Arrays.asList() for
  single-item collections
* StringBuilder instead of StringBuffer
* String concatenation instead of straight-forward StringBuilder
  (i.e. non-conditional append() calls)
* typos: hierarchchical -> hierarchical, intends -> indents
* drop unused parameter from
  AbstractAttributesProcessor::processAttributes()
* when processing only values in a map, use .values() rather than
  .entrySet()
* split StringBuilder::append(a + b) into two ::append() calls
* clean up the method parameter construction in MethodSerializer (to
  avoid deleting the ", " at the end)

Fixes in generated code:
* use { } as requested the Checkstyle rules
* handle empty service interface sets explicitly

It would be nice to use the <> operator without specifying the type
where possible, but the Eclipse AST parser used in the unit tests is
too old for this (so the generated code is correct and works
elsewhere, but fails the unit tests).

Change-Id: I725ce8d98f9ee389d394772733663ecacaccf8fa
Signed-off-by: Stephen Kitt <skitt@redhat.com>