This should cover:
```
library "lib" api;
import Foo library default;
import library default;
import library "lib";
```
This splits out `PackageName` and `LibraryName` to their own parse nodes
so that checking can ignore them and still get a balanced parse tree
(otherwise, we essentially need to implement handling of the parse nodes
only to remove the identifiers/string literals -- the optional names
mean we can't blindly do that as before). For reference, these nodes
don't need to be handled because CheckParseTree will need to directly
funnel import information along with checked IRs.
This detects ordering issues with the `package` and `import` statements.
`library` is changed from package-specific to instead be generic between
the two, since structurally it's non-specific.
The next step would be to start exposing the results for the driver to
make ordering decisions for checking. That'll involve further
modifications to this code, but this felt like a reasonable change point
because it's the extent of the parser enforcement, and still causes
significant refactoring.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Jon Ross-Perkins
