Provide method for filtering stdlib from PackageTree imports

[sdboyer]

Working on the new dep tool has made it quite painfully clear that the PackageTree methods are really, really narrowly tailored to the solver's needs.

For one - stdlib imports are included in the returns from ListPackages(), and there's no way for dependers to pull them out themselves. In a sense, this is intentional - I'm not fully comfortable with my strategy of just literally listing out the stdlib packages (and others) in source code, so I didn't want to expose it publicly. But it's crunch time now, and it seems pretty clear that a more comprehensive solution is quite involved, so it's time to expose that information.

[sdboyer]

Note - the underpinnings here are a little different, now:

I'm not fully comfortable with my strategy of just literally listing out the stdlib packages (and others) in source code,

That hardcoded list no longer exists. It's been replaced with an isStdlib() func (#118) that's used to determine if an import path points to stdlib (and is the same implementation as what the go toolchain uses).