Currently go/loader looks for packages relative to $GOPATH and $GOROOT but it should
also allow paths relative to the current directory such as ./foo, and do so in a manner
consistent with the 'go' tool.

This is a subtle change because the name supplied by the user (./foo) may not match the
canonical import path ("bar/foo") that is widely used as a map key, so we'll
need to audit and test the code thoroughly.

Do this after the 1.4 tree thaw.