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

This pull request uses go.tools/importer and removes the dependency on honnef.co/go/importer. At the same time, parsing is greatly simplified, because go.tools/importer is doing everything for us.

This change affects errcheck's behaviour in two ways:

1. It can no longer use the gc importer for packages in GOROOT, which means it will have to parse the standard library on every run. This does add a small increase in run time and CPU usage. I don't see any way around this currently. `importer.SourceLoader` is required to return `[]*ast.File`, which implies parsing. And the documentation on `Context.Loader`, it being nil and that causing the gc importer to be used is irrelevant because the only way to import packages, `(*Importer).LoadPackage()` requires `Context.Loader` to be `!= nil`.

2. Programs using cgo fail differently now. Instead of failing on the `import "C"`, it will now skip over all Go files that use cgo, which means identifiers will be missing. Using those will lead to errors like `DestroyDatabase not declared by package levigo`.

Closes gh-26
Updates gh-16