Add 'halves()' to path interface

Adds a func to the path interface which breaks up the path
into its two distinct halves: the prefix builder, and the folder
and item elements.  Likely to be useful in the future where
prefixes will be of arbitrary length, and downstream consumers
are primarly focused on the non-prefix portion of the path.

src/internal/operations/pathtransformer/restore_path_transformer_test.go

@@ -36,12 +36,8 @@ func (suite *RestorePathTransformerUnitSuite) TestGetPaths() {
 		repoRef path.Path,
 		unescapedFolders ...string,
 	) string {
-		return path.Builder{}.
-			Append(
-				repoRef.Tenant(),
-				repoRef.Service().String(),
-				repoRef.ResourceOwner(),
-				repoRef.Category().String()).
+		pfx, _ := repoRef.Halves()
+		return pfx.
 			Append(unescapedFolders...).
 			String()
 	}

src/pkg/path/path.go

@@ -115,6 +115,10 @@ type Path interface {
 	ShortRef() string
 	// ToBuilder returns a Builder instance that represents the current Path.
 	ToBuilder() *Builder
+	// Halves breaks the path into its prefix (tenant, services, resources, category)
+	// and suffix (all parts after the prefix).  If either half is empty, that half
+	// returns an empty, non-nil, value.
+	Halves() (*Builder, Elements)
 
 	// Every path needs to comply with these funcs to ensure that PII
 	// is appropriately hidden from logging, errors, and other outputs.

src/pkg/path/resource_path.go

@@ -156,3 +156,17 @@ func (rp dataLayerResourcePath) ToBuilder() *Builder {
 func (rp *dataLayerResourcePath) UpdateParent(prev, cur Path) bool {
 	return rp.Builder.UpdateParent(prev.ToBuilder(), cur.ToBuilder())
 }
+
+func (rp *dataLayerResourcePath) Halves() (*Builder, Elements) {
+	pfx, sfx := &Builder{}, Elements{}
+
+	b := rp.Builder
+	if len(b.elements) > 0 {
+		lenPfx := 2 + (len(rp.serviceResources) * 2)
+
+		pfx = &Builder{elements: append(Elements{}, b.elements[:lenPfx]...)}
+		sfx = append(Elements{}, b.elements[lenPfx-1:]...)
+	}
+
+	return pfx, sfx
+}

src/pkg/path/resource_path_test.go

@@ -678,3 +678,57 @@ func (suite *PopulatedDataLayerResourcePath) TestUpdateParent_NoopsNils() {
 		})
 	}
 }
+
+func (suite *PopulatedDataLayerResourcePath) TestHalves() {
+	t := suite.T()
+
+	onlyPrefix, err := path.BuildPrefix(
+		"titd",
+		[]path.ServiceResource{{path.ExchangeService, "pr"}},
+		path.ContactsCategory)
+	require.NoError(t, err, clues.ToCore(err))
+
+	fullPath, err := path.Build(
+		"tid",
+		[]path.ServiceResource{{path.ExchangeService, "pr"}},
+		path.ContactsCategory,
+		true,
+		"fld", "item")
+	require.NoError(t, err, clues.ToCore(err))
+
+	table := []struct {
+		name      string
+		dlrp      path.Path
+		expectPfx *path.Builder
+		expectSfx path.Elements
+	}{
+		{
+			name: "only prefix",
+			dlrp: onlyPrefix,
+			expectPfx: path.Builder{}.Append(
+				"tid",
+				path.ExchangeService.String(),
+				"pr",
+				path.ContactsCategory.String()),
+			expectSfx: path.Elements{},
+		},
+		{
+			name: "full path",
+			dlrp: fullPath,
+			expectPfx: path.Builder{}.Append(
+				"tid",
+				path.ExchangeService.String(),
+				"pr",
+				path.ContactsCategory.String()),
+			expectSfx: path.Elements{"foo", "bar"},
+		},
+	}
+	for _, test := range table {
+		suite.Run(test.name, func() {
+			t := suite.T()
+			pfx, sfx := test.dlrp.Halves()
+			assert.Equal(t, test.expectPfx, pfx, "prefix")
+			assert.Equal(t, test.expectSfx, sfx, "suffix")
+		})
+	}
+}