Handle themes in the new file cache (for images, assets)

In the newly consolidated file cache implementation, we forgot that we also look in the theme(s) for assets (SCSS transformations etc.), which is not good for Netlify and the demo sites.

Fixes #5460

hugolib/filesystems/basefs.go

@@ -81,6 +81,7 @@ type SourceFilesystems struct {
 	Layouts    *SourceFilesystem
 	Archetypes *SourceFilesystem
 	Assets     *SourceFilesystem
+	Resources  *SourceFilesystem
 
 	// This is a unified read-only view of the project's and themes' workdir.
 	Work *SourceFilesystem
@@ -375,6 +376,13 @@ func (b *sourceFilesystemsBuilder) Build() (*SourceFilesystems, error) {
 	}
 	b.result.Assets = sfs
 
+	sfs, err = b.createFs(true, false, "resourceDir", "resources")
+	if err != nil {
+		return nil, err
+	}
+
+	b.result.Resources = sfs
+
 	sfs, err = b.createFs(false, true, "", "")
 	if err != nil {
 		return nil, err

hugolib/filesystems/basefs_test.go

@@ -108,6 +108,7 @@ theme = ["atheme"]
 	checkFileCount(bfs.Data.Fs, "", assert, 9)        // 7 + 2 themes
 	checkFileCount(bfs.Archetypes.Fs, "", assert, 10) // 8 + 2 themes
 	checkFileCount(bfs.Assets.Fs, "", assert, 9)
+	checkFileCount(bfs.Resources.Fs, "", assert, 10)
 	checkFileCount(bfs.Work.Fs, "", assert, 78)
 
 	assert.Equal([]string{filepath.FromSlash("/my/work/mydata"), filepath.FromSlash("/my/work/themes/btheme/data"), filepath.FromSlash("/my/work/themes/atheme/data")}, bfs.Data.Dirnames)
@@ -228,6 +229,8 @@ func TestRealDirs(t *testing.T) {
 	assert.Equal(filepath.Join(root, "myassets/scss"), realDirs[0])
 	assert.Equal(filepath.Join(themesDir, "mytheme/assets/scss"), realDirs[len(realDirs)-1])
 
+	checkFileCount(bfs.Resources.Fs, "", assert, 3)
+
 	assert.NotNil(bfs.themeFs)
 	fi, b, err := bfs.themeFs.(afero.Lstater).LstatIfPossible(filepath.Join("resources", "t1.txt"))
 	assert.NoError(err)