replace stub isStdLib()

[jessfraz]

closes #20

[freeformz]

This is the same function that the go tool uses when expanding "std" to all stdlib packages.

[sdboyer]

huh. that's pretty cool. LGTM

i've been imagining in some possible future where we have a central-ish server that we'd use some special naming scheme - e.g. p/<user>/<project>, or something, which would necessitate revising this. but, if and when we get there, it's easy to make an exception for that here :)