[D] Support looking up symbols in the current and imported modules.

[D] Support looking up symbols in the current and imported modules.

In D, there is the notion of modules, and importing from one to the other,
whether it is a basic, selective or renamed import declaration.

	module A;
	import X;
	void foo() {
	  import Y : bar;
	}

If the compiler emits DW_TAG_imported_declaration at the appropriate locations,
then we can make use of what gdb stores in using_direct when performing
nonlocal symbol lookups.

gdb/ChangeLog

	* Makefile.in (SFILES): Add d-namespace.c.
	(COMMON_OBS): Add d-namespace.o.
	* d-lang.c (d_language_defn): Use d_lookup_symbol_nonlocal as the
	la_lookup_symbol_nonlocal callback function pointer.
	* d-lang.h (d_lookup_symbol_nonlocal): New declaration.
	(d_lookup_nested_symbol): New declaration.
	* d-namespace.c: New file.