gdb: rename partial_symtab::read_dependencies to expand_dependencies

This method calls partial_symtab::expand_psymtab on all dependencies of
a psymtab.  Given that there is also a partial_symtab::read_symtab
method, I think it would be clearer to name this function
expand_dependencies, rather than read_dependencies.

gdb/ChangeLog:

	* psympriv.h (partial_symtab) <read_dependencies>: Rename to...
	<expand_dependencies>: ... this.
	* psymtab.c (partial_symtab::read_dependencies): Rename to...
	(partial_symtab::expand_dependencies): ... this.
	* dwarf2/read.c (dwarf2_include_psymtab) <expand_psymtab>:
	Update.
	(dwarf2_psymtab::expand_psymtab): Update.
	* dbxread.c (dbx_psymtab_to_symtab_1): Update.
	* mdebugread.c (psymtab_to_symtab_1): Update.
	* xcoffread.c (xcoff_psymtab_to_symtab_1): Update.