Introduce RepoSpec.

This PR closes a TODO in the fileLoader type to
replace the string representing a git repo url
with a struct called holding validated fields.

New struct is called RepoSpec.

It holds what we need in coming PRs for remote
repo base containment checks, and will also allow
the overlay cycle prevention code to take into
account different host schemes or branches.

The struct is in a new file called repospec.go.
The new file is mostly just a code move of url
parsing code that was in cloner.go.  Git "blame"
history preserved through the move.

No change in execution (see final commit by itself).

pkg/loader/fileloader.go

@@ -86,9 +86,9 @@ type fileLoader struct {
 	// The Load function reads from this directory,
 	// or directories below it.
 	root fs.ConfirmedDir
-	// URI, if any, used for a download into root.
-	// TODO(monopole): use non-string type.
-	uri string
+	// If this is non-nil, the files were
+	// obtained from the given repository.
+	repoSpec *git.RepoSpec
 	// File system utilities.
 	fSys fs.FileSystem
 	// Used to clone repositories.
@@ -179,26 +179,26 @@ func (l *fileLoader) New(path string) (ifc.Loader, error) {
 func newLoaderAtGitClone(
 	uri string, fSys fs.FileSystem,
 	referrer *fileLoader, cloner git.Cloner) (ifc.Loader, error) {
-	tmpDirForRepo, pathInRepo, err := cloner(uri)
+	repoSpec, err := cloner(uri)
 	if err != nil {
 		return nil, err
 	}
-	root, f, err := fSys.CleanedAbs(
-		filepath.Join(tmpDirForRepo, pathInRepo))
+	root, f, err := fSys.CleanedAbs(repoSpec.AbsPath())
 	if err != nil {
 		return nil, err
 	}
 	if f != "" {
 		return nil, fmt.Errorf(
-			"'%s' refers to file '%s'; expecting directory", pathInRepo, f)
+			"'%s' refers to file '%s'; expecting directory",
+			repoSpec.AbsPath(), f)
 	}
 	return &fileLoader{
 		root:     root,
 		referrer: referrer,
-		uri:      uri,
+		repoSpec: repoSpec,
 		fSys:     fSys,
 		cloner:   cloner,
-		cleaner:  func() error { return fSys.RemoveAll(tmpDirForRepo) },
+		cleaner:  repoSpec.Cleaner(fSys),
 	}, nil
 }
 
@@ -221,11 +221,12 @@ func (l *fileLoader) errIfArgEqualOrHigher(
 // I.e. Allow a distinction between git URI with
 // path foo and tag bar and a git URI with the same
 // path but a different tag?
+// TODO(monopole): Use parsed data instead of looking at Raw().
 func (l *fileLoader) errIfPreviouslySeenUri(uri string) error {
-	if strings.HasPrefix(l.uri, uri) {
+	if strings.HasPrefix(l.repoSpec.Raw(), uri) {
 		return fmt.Errorf(
 			"cycle detected: URI '%s' referenced by previous URI '%s'",
-			uri, l.uri)
+			uri, l.repoSpec.Raw())
 	}
 	if l.referrer == nil {
 		return nil