extendSchema: Use consistent naming + inline 'getExtendedType' func

[IvanGoncharov]

Based on this discussion: #1322 (comment)
I think naming should be consistent with types and predicates:

graphql-js/src/type/definition.js

Lines 52 to 63 in ae5b163

	export function isType(type: mixed): boolean %checks {
	return (
	isScalarType(type) ||
	isObjectType(type) ||
	isInterfaceType(type) ||
	isUnionType(type) ||
	isEnumType(type) ||
	isInputObjectType(type) ||
	isListType(type) ||
	isNonNullType(type)
	);
	}

graphql-js/src/type/definition.js

Lines 462 to 471 in ae5b163

	export function isNamedType(type: mixed): boolean %checks {
	return (
	isScalarType(type) ||
	isObjectType(type) ||
	isInterfaceType(type) ||
	isUnionType(type) ||
	isEnumType(type) ||
	isInputObjectType(type)
	);
	}

[leebyron]

Makes sense to me - do you mind rebasing?

[IvanGoncharov]

@leebyron Rebased

[mjmahone]

Thank you!