go/types: better names for internal helper functions (cleanup)

Internal helper functions for type-checking type expressions were
renamed to make it clearer when they should be used:

	typExpr (w/o def) -> typ
	typExpr (w/  def) -> definedType
	typ               -> indirectType
	typExprInternal   -> typInternal

The rename emphasizes that in most cases Checker.typ should be used
to compute the types.Type from an ast.Type. If the type is defined,
definedType should be used. For composite type elements which are
not "inlined" in memory, indirectType should be used.

In the process, implicitly changed several uses of indirectType
(old: typ) to typ (old: typExpr) by not changing the respective
function call source. These implicit changes are ok in those
places because either call is fine where we are not concerned
about composite type elements. But using typ (old: typExpr) is
more efficient than using indirectType (old: typ).

Change-Id: I4ad14d5357c5f94b6f1c33173de575c4cd05c703
Reviewed-on: https://go-review.googlesource.com/130595


Reviewed-by: Alan Donovan <adonovan@google.com>