Add documentation around catalogers, UI elements, and the event bus (#143)

* add basic documentation for catalogers (with refactoring for simplification)

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* add docs for catalog parsers, UI, and event bus

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

* update bus phrasing

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

syft/cataloger/python/cataloger.go

@@ -7,14 +7,13 @@ import (
 	"github.com/anchore/syft/syft/scope"
 )
 
+// Cataloger catalogs pkg.WheelPkg, pkg.EggPkg, and pkg.PythonRequirementsPkg Package Types defined in Python ecosystem files.
 type Cataloger struct {
 	cataloger common.GenericCataloger
 }
 
-func NewCataloger() *Cataloger {
-	// we want to match on partial dir names
-	// 	/home/user/requests-2.10.0.dist-info/METADATA
-	//	/home/user/requests-2.10.0/dist-info/METADATA
+// New returns a new Python cataloger object.
+func New() *Cataloger {
 	globParsers := map[string]common.ParserFn{
 		"**/*egg-info/PKG-INFO":  parseEggMetadata,
 		"**/*dist-info/METADATA": parseWheelMetadata,
@@ -27,14 +26,17 @@ func NewCataloger() *Cataloger {
 	}
 }
 
+// Name returns a string that uniquely describes this cataloger.
 func (a *Cataloger) Name() string {
 	return "python-cataloger"
 }
 
+// SelectFiles returns a set of discovered Python ecosystem files from the user content source.
 func (a *Cataloger) SelectFiles(resolver scope.FileResolver) []file.Reference {
 	return a.cataloger.SelectFiles(resolver)
 }
 
+// Catalog returns the Packages indexed from all Python ecosystem files discovered.
 func (a *Cataloger) Catalog(contents map[file.Reference]string) ([]pkg.Package, error) {
 	return a.cataloger.Catalog(contents, a.Name())
 }