[Ada] Read directory in Ada.Directories.Start_Search rather than Get_Next_Entry

[Ada] Read directory in Ada.Directories.Start_Search rather than Get_Next_Entry

gcc/ada/

	* libgnat/a-direct.adb (Search_Data): Remove type.
	(Directory_Vectors): New package instantiation.
	(Search_State): New type.
	(Fetch_Next_Entry): Remove.
	(Close): Remove.
	(Finalize): Rewritten.
	(Full_Name): Ditto.
	(Get_Next_Entry): Return next entry from Search results vector
	rather than querying the directory directly using readdir.
	(Kind): Rewritten.
	(Modification_Time): Rewritten.
	(More_Entries): Use Search state cursor to determine if more
	entries are available for users to read.
	(Simple_Name): Rewritten.
	(Size): Rewritten.
	(Start_Search_Internal): Rewritten to load the contents of the
	directory that matches the pattern and filter into the search
	object.
	* libgnat/a-direct.ads (Search_Type): New type.
	(Search_Ptr): Ditto.
	(Directory_Entry_Type): Rewritten to support new Start_Search
	procedure.
	* libgnat/s-filatt.ads (File_Length_Attr): New function.