minimal test case for go/types import

Steps to reproduce:

	1. git clone https://github.com/kr/gotypesrepro $GOPATH/src/p
	2. cd $GOPATH/src/p
	3. go run try.go # fails
	4. go install p
	5. go run try.go # now it works

Expected output:

	package testdata ("testdata") # first go run
	package testdata ("testdata") # second go run

Actual output:

	testdata/import.go:3:8: could not import p (can't find import: p)
	package testdata ("testdata")