Use go.tools/importer to parse and import packages a la go build

This removes the dependency on honnef.co/go/importer
remove duplicate parsing

We were parsing code twice: once via go.tools/importer and once
manually.
clarrify "no go files" error
Merge pull request #27 from dominikh/gh-26

Use go.tools/importer instead of honnef.co/go/importer