Avoid comparing lexical positions across units for package imports

Package imports already resolve names declared in packages from other
compilation units. After `NetScope::find_import()` finds such an import,
however, `symbol_search()` continues with an unbound lookup. It then compares
the declaration position in the package with the reference position in the
importing unit. Lexical positions are local to a compilation unit, so these
unrelated values can make a legal imported variable, named event, or parameter
appear to be used before its declaration.

Set `scope_is_bound` after `NetScope::find_import()` has checked that the
import is visible at the reference. The import already identifies the package
that declares the name, so binding the search avoids the invalid cross-unit
comparison and prevents lookup from continuing outside that package.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

symbol_search.cc

@@ -374,7 +374,11 @@ bool symbol_search(const LineInfo*li, Design*des, NetScope*scope,
 	    // Imports are not visible through hierachical names
 	    if (auto import_scope = scope->find_import(
 		      des, path_tail.name, lexical_pos)) {
+		    // The import binds the name to the returned package. Continue
+		    // with the search bound to it, since lexical positions are
+		    // unit-local.
 		  scope = import_scope;
+		  scope_is_bound = true;
 		  continue;
 	    }