Move dwarf2_get_dwz_file to dwarf2/dwz.h

Move dwarf2_get_dwz_file to dwarf2/dwz.h

This moves dwarf2_get_dwz_file and some helper code to dwarf2/dwz.h.
The main benefit of this is just shrinking dwarf2/read.c a little bit.

gdb/ChangeLog
2021-03-06  Tom Tromey  <tom@tromey.com>

	* dwarf2/sect-names.h (dwarf2_elf_names): Declare.
	* dwarf2/read.h (dwarf2_get_dwz_file): Move to dwz.h.
	* dwarf2/read.c (dwarf2_elf_names): No longer static.
	(locate_dwz_sections, dwz_search_other_debugdirs)
	(dwarf2_get_dwz_file): Move to dwz.c.
	* dwarf2/dwz.h (dwarf2_get_dwz_file): Move declaration from
	read.h.
	* dwarf2/dwz.c (locate_dwz_sections, dwz_search_other_debugdirs)
	(dwarf2_get_dwz_file): Move from read.c.