Fix codegen for Go Modules

Fix two issues with codegen that cause it not to work with Go Modules.

1. When parsing the code files the package SrcRoot and ImportPath were
being joined which assumes the package is hosted inside a GOPATH src
directory. Using those two fields is unnecessary because a package also
knows what directory it lives in, which is in the Dir field. This change
switches to using the Dir field.

2. When checking the types in the source files the default importer is
being used. Unfortunately the default importer has not been updated to
work with Go Modules and it only looks in the GOPATH for imports.
Luckily the Go team were convinced to backport some logic from the newer
go/packages package to go/build, and we can make use of the go/build
importer which supports Go Modules. One downside of using the go/build
importer is it is said to be slow. For this size project though it's I
don't think it's a problem.

In the future codegen should be rewritten to use go/packages once it is
included in the stdlib. go/packages currently lives at
golang.org/x/tools/go/packages. Once it is in the stdlib I believe it
will replace go/build.

Both fixes are compatible with Go 1.13, Go 1.12, Go 1.11, Go 1.10.

I've also updated the TravisCI config to run go generate, and to add the
newer versions of Go to demonstrate that this works on all versions of
Go.

_codegen/main.go

@@ -181,7 +181,7 @@ func parsePackageSource(pkg string) (*types.Scope, *doc.Package, error) {
 	files := make(map[string]*ast.File)
 	fileList := make([]*ast.File, len(pd.GoFiles))
 	for i, fname := range pd.GoFiles {
-		src, err := ioutil.ReadFile(path.Join(pd.SrcRoot, pd.ImportPath, fname))
+		src, err := ioutil.ReadFile(path.Join(pd.Dir, fname))
 		if err != nil {
 			return nil, nil, err
 		}
@@ -194,7 +194,7 @@ func parsePackageSource(pkg string) (*types.Scope, *doc.Package, error) {
 	}
 
 	cfg := types.Config{
-		Importer: importer.Default(),
+		Importer: importer.ForCompiler(fset, "source", nil),
 	}
 	info := types.Info{
 		Defs: make(map[*ast.Ident]types.Object),