Panic in gta for ImportPath "."

Hi,

Thanks for creating this project!

Unfortunately with the recent changes to optimise gta processing speed, we've come across a panic whilst running gta on one of our repos. I'm afraid I'm not allowed to share the repo.

The error output is:
```go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x606721]

goroutine 1 [running]:
github.com/digitalocean/gta.(*GTA).ChangedPackages(0xc0000c3310, 0x3, 0x4, 0xc0000c3310)
/root/gta/gta.go:187 +0x201
main.main()
/root/gta/cmd/gta/main.go:80 +0x6ef
r
```

I have tracked the issue down to:
* Change in commit: https://github.com/digitalocean/gta/commit/d5dac295d3625c15402e57fbd35f32fc0b0f7c2b
* Actual error is: `no buildable Go source files in .`
* The error is not trapped through any error detection and eventually panics in https://github.com/digitalocean/gta/blob/master/gta.go#L193 when a call to pkg.ImportPath is made but pkg is nil.
* Specifically:
```go
if _, ok := p.forward[importPath]; !ok {
// TODO(bc): importPath is probably _not_ the right thing to use for Dir. So what _is_?
return nil, &build.NoGoError{Dir: importPath}
}
```
* Path `.` is not in the p.forward list

I'm afraid I'm not sure why there is `.` in the p.forward map.

Any help would be gratefully received!