Major rework of codebase

- Get rid of 'core' and move CLI to cmd/gas directory
- Migrate (most) tests to use Ginkgo and testutils framework
- GAS now expects package to reside in $GOPATH
- GAS now can resolve dependencies for better type checking (if package
  on GOPATH)
- Simplified public API

testutils/pkg.go

@@ -0,0 +1,132 @@
+package testutils
+
+import (
+	"fmt"
+	"go/build"
+	"go/parser"
+	"io/ioutil"
+	"log"
+	"os"
+	"path"
+	"strings"
+
+	"github.com/GoASTScanner/gas"
+	"golang.org/x/tools/go/loader"
+)
+
+type buildObj struct {
+	pkg     *build.Package
+	config  loader.Config
+	program *loader.Program
+}
+
+type TestPackage struct {
+	Path   string
+	Files  map[string]string
+	ondisk bool
+	build  *buildObj
+}
+
+// NewPackage will create a new and empty package. Must call Close() to cleanup
+// auxilary files
+func NewTestPackage() *TestPackage {
+	// Files must exist in $GOPATH
+	sourceDir := path.Join(os.Getenv("GOPATH"), "src")
+	workingDir, err := ioutil.TempDir(sourceDir, "gas_test")
+	if err != nil {
+		return nil
+	}
+
+	return &TestPackage{
+		Path:   workingDir,
+		Files:  make(map[string]string),
+		ondisk: false,
+		build:  nil,
+	}
+}
+
+// AddFile inserts the filename and contents into the package contents
+func (p *TestPackage) AddFile(filename, content string) {
+	p.Files[path.Join(p.Path, filename)] = content
+}
+
+func (p *TestPackage) write() error {
+	if p.ondisk {
+		return nil
+	}
+	for filename, content := range p.Files {
+		if e := ioutil.WriteFile(filename, []byte(content), 0644); e != nil {
+			return e
+		}
+	}
+	p.ondisk = true
+	return nil
+}
+
+// Build ensures all files are persisted to disk and built
+func (p *TestPackage) Build() error {
+	if p.build != nil {
+		return nil
+	}
+	if err := p.write(); err != nil {
+		return err
+	}
+	basePackage, err := build.Default.ImportDir(p.Path, build.ImportComment)
+	if err != nil {
+		return err
+	}
+
+	packageConfig := loader.Config{Build: &build.Default, ParserMode: parser.ParseComments}
+	packageFiles := make([]string, 0)
+	for _, filename := range basePackage.GoFiles {
+		packageFiles = append(packageFiles, path.Join(p.Path, filename))
+	}
+
+	packageConfig.CreateFromFilenames(basePackage.Name, packageFiles...)
+	program, err := packageConfig.Load()
+	if err != nil {
+		return err
+	}
+	p.build = &buildObj{
+		pkg:     basePackage,
+		config:  packageConfig,
+		program: program,
+	}
+	return nil
+}
+
+// CreateContext builds a context out of supplied package context
+func (p *TestPackage) CreateContext(filename string) *gas.Context {
+	if err := p.Build(); err != nil {
+		log.Fatal(err)
+		return nil
+	}
+
+	for _, pkg := range p.build.program.Created {
+		for _, file := range pkg.Files {
+			pkgFile := p.build.program.Fset.File(file.Pos()).Name()
+			strip := fmt.Sprintf("%s%c", p.Path, os.PathSeparator)
+			pkgFile = strings.TrimPrefix(pkgFile, strip)
+			if pkgFile == filename {
+				ctx := &gas.Context{
+					FileSet: p.build.program.Fset,
+					Root:    file,
+					Config:  gas.NewConfig(),
+					Info:    &pkg.Info,
+					Pkg:     pkg.Pkg,
+					Imports: gas.NewImportTracker(),
+				}
+				ctx.Imports.TrackPackages(ctx.Pkg.Imports()...)
+				return ctx
+			}
+		}
+	}
+	return nil
+}
+
+// Close will delete the package and all files in that directory
+func (p *TestPackage) Close() {
+	if p.ondisk {
+		os.RemoveAll(p.Path)
+	}
+}