Allow scanning unpacked container filesystems (#1485)

* source: avoid second-step of symlink resolution in directory resolver

We can use the already existing file tree to peform symlink resolution
for FilesByPath, instead of traversing the symlinks again.

This moves all of the symlink logic into the indexing code, and then we
can rely on syft's resolution algorithm over the index in this part of
the codebase.

Signed-off-by: Justin Chadwell <me@jedevc.com>

* source: add base parameter to directory resolver

The new base parameter is an optional parameter for the directory
resolver that resolves all symlinks relative to this root. There are two
intended use cases:

- base = "/". The previous behavior, symlinks are resolved relative to
the root filesystem.
- base = path. Symlinks are resolved relative to the target filesystem,
allowing correct behavior when scanning unpacked container filesystems
on disk.

Signed-off-by: Justin Chadwell <me@jedevc.com>

* source: add tests for new base parameter

Signed-off-by: Justin Chadwell <me@jedevc.com>

---------

Signed-off-by: Justin Chadwell <me@jedevc.com>

syft/source/source.go

@@ -32,6 +32,7 @@ type Source struct {
 	Metadata          Metadata
 	directoryResolver *directoryResolver `hash:"ignore"`
 	path              string
+	base              string
 	mutex             *sync.Mutex
 	Exclusions        []string `hash:"ignore"`
 }
@@ -252,6 +253,11 @@ func NewFromDirectory(path string) (Source, error) {
 	return NewFromDirectoryWithName(path, "")
 }
 
+// NewFromDirectory creates a new source object tailored to catalog a given filesystem directory recursively.
+func NewFromDirectoryRoot(path string) (Source, error) {
+	return NewFromDirectoryRootWithName(path, "")
+}
+
 // NewFromDirectoryWithName creates a new source object tailored to catalog a given filesystem directory recursively, with an explicitly provided name.
 func NewFromDirectoryWithName(path string, name string) (Source, error) {
 	s := Source{
@@ -267,6 +273,23 @@ func NewFromDirectoryWithName(path string, name string) (Source, error) {
 	return s, nil
 }
 
+// NewFromDirectoryRootWithName creates a new source object tailored to catalog a given filesystem directory recursively, with an explicitly provided name.
+func NewFromDirectoryRootWithName(path string, name string) (Source, error) {
+	s := Source{
+		mutex: &sync.Mutex{},
+		Metadata: Metadata{
+			Name:   name,
+			Scheme: DirectoryScheme,
+			Path:   path,
+			Base:   path,
+		},
+		path: path,
+		base: path,
+	}
+	s.SetID()
+	return s, nil
+}
+
 // NewFromFile creates a new source object tailored to catalog a file.
 func NewFromFile(path string) (Source, func()) {
 	return NewFromFileWithName(path, "")
@@ -428,7 +451,7 @@ func (s *Source) FileResolver(scope Scope) (FileResolver, error) {
 			if err != nil {
 				return nil, err
 			}
-			resolver, err := newDirectoryResolver(s.path, exclusionFunctions...)
+			resolver, err := newDirectoryResolver(s.path, s.base, exclusionFunctions...)
 			if err != nil {
 				return nil, fmt.Errorf("unable to create directory resolver: %w", err)
 			}