By default an identifier that has been imported into a package is not
available for imports by other packages. Only imports that have been
exported can be imported again. E.g.
```
package P1;
int x;
endpackage
package P2;
import P1::x;
export P1::x;
endpackage
module test;
import P2::x; // This will only work if x has been exported.
endmodule
```
Exports follow the same syntax as imports and allow both export of specific
identifiers or wildcard export. Export supports the special `*::*` target,
which will export all imported items.
Add support for handling package exports.
There is one special cases that needs to be considered. Usually when using
wildcard imports from multiple packages it is an error if there multiple
import candidates for an identifier. With exports it is possible that there
are multiple candidates through different packets, but they all refer to
the same identifier. In this case it does not create a conflict. E.g.
```
package P1;
int x;
endpackage
package P2;
import P1::x;
export P1::x;
endpackage
package P3;
import P1::*;
import P2::*;
int y = x; // No import conflict
endpackage
```
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cary R
Lars-Peter Clausen
Stephen Williams