a68: fix deduplication of imported modes

a68: fix deduplication of imported modes

The internal global list of modes maintained by the compiler should
not contain two modes that are equivalent.  When importing module
interfaces, the modes in these interfaces should get deduplicated
before being "interned" in the compiler's modes list.  This commit
fixes the deduplication to accommodate the fact that more than one
module interface may be read from a given packet (compilation unit)
and also that multiple interfaces may be imported indirectly via
publicized modules.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* a68-imports.cc (a68_decode_modes): Do not deduplicate imported
	modes here.
	(a68_open_packet): Do it here.
	* a68-parser-extract.cc (extract_revelation): Recurse into
	publicized modules after interning modes in the current module,
	not before.

gcc/testsuite/ChangeLog

	* algol68/execute/modules/module23bar.a68: New test.
	* algol68/execute/modules/module23foo.a68: Likewise.
	* algol68/execute/modules/program-23.a68: Likewise.