Ensure Sass $govuk-* URL context variables are always set

This gives internal pages their own `$govuk-assets-path` Sass variable to internal GOV.UK Frontend to align with Nunjucks

Whilst plugin Sass can continue to use `/plugin-assets`

lib/assets/sass/manage-prototype.scss

@@ -1,5 +1,3 @@
-$govuk-assets-path: '/manage-prototype/dependencies/govuk-frontend/govuk/assets/';
-
 // Import GOV.UK Frontend within the kit dependency
 @import ".tmp/sass/kit-frontend-dependency";
 

lib/build.js

@@ -89,12 +89,18 @@ function sassInclude (filePath) {
 
 function sassKitFrontendDependency () {
   const timer = startPerformanceTimer()
+
   const internalGovUkFrontendDir = getInternalGovukFrontendDir()
   const internalGovUkFrontendConfig = fse.readJsonSync(path.join(internalGovUkFrontendDir, 'govuk-prototype-kit.config.json'))
-  const fileContents = internalGovUkFrontendConfig.sass
+
+  const govukFrontendSass = internalGovUkFrontendConfig.sass
     .map(sassPath => path.join(internalGovUkFrontendDir, sassPath))
-    .map(sassInclude)
-    .join('\n')
+
+  const fileContents = sassVariables('/manage-prototype/dependencies') +
+    govukFrontendSass
+      .map(sassInclude)
+      .join('\n')
+
   ensureTempDirExists(tmpSassDir)
   fse.writeFileSync(path.join(tmpSassDir, '_kit-frontend-dependency.scss'), fileContents)
   endPerformanceTimer('sassKitFrontendDependency', timer)
@@ -116,37 +122,52 @@ function sassLegacyPatterns () {
       .join('\n')
   } else {
     fileContents = `
-/* Legacy patterns not included as govuk-frontend plugin not installed */    
+/* Legacy patterns not included as govuk-frontend plugin not installed */
     `
   }
   fse.writeFileSync(path.join(tmpSassDir, '_legacy-patterns.scss'), fileContents)
   endPerformanceTimer('sassLegacyPatterns', timer)
 }
 
-function sassPlugins () {
-  const timer = startPerformanceTimer()
+function sassVariables (contextPath = '', isLegacyGovukFrontend = false) {
   let fileContents = ''
+
   // Keep $govuk-extensions-url-context for backwards compatibility
   // TODO: remove in v14
-  fileContents += '$govuk-extensions-url-context: "/plugin-assets";\n'
-  fileContents += '$govuk-plugins-url-context: "/plugin-assets";\n'
+  fileContents += `$govuk-extensions-url-context: "${contextPath}";\n`
+  fileContents += `$govuk-plugins-url-context: "${contextPath}";\n`
   fileContents += '$govuk-prototype-kit-major-version: 13;\n'
-  if (plugins.legacyGovukFrontendFixesNeeded()) {
+
+  // Patch missing 'init.scss' before GOV.UK Frontend v4.4.0
+  // in plugin versions, but will default to false for internal
+  if (isLegacyGovukFrontend) {
     fileContents += '$govuk-global-styles: true !default;\n'
     fileContents += '$govuk-new-link-styles: true !default;\n'
   }
-  fileContents += plugins.getFileSystemPaths('sass')
-    .map(sassInclude)
-    .join('\n')
+
+  return fileContents
+}
+
+function sassPlugins () {
+  const timer = startPerformanceTimer()
+
+  const fileContents = sassVariables('/plugin-assets', plugins.legacyGovukFrontendFixesNeeded()) +
+    plugins.getFileSystemPaths('sass')
+      .map(sassInclude)
+      .join('\n')
+
   ensureTempDirExists(tmpSassDir)
   fse.writeFileSync(path.join(tmpSassDir, '_plugins.scss'), fileContents)
   endPerformanceTimer('sassPlugins', timer)
 }
 
 function sassErrorPage () {
   const timer = startPerformanceTimer()
+
+  const fileContents = sassVariables('/manage-prototype/dependencies') +
+    sassInclude(path.join(libSassDir, 'includes', '_error-page.scss'))
+
   ensureTempDirExists(tmpSassDir)
-  const fileContents = sassInclude(path.join(libSassDir, 'includes', '_error-page.scss'))
   fse.writeFileSync(path.join(tmpSassDir, 'error-page.scss'), fileContents)
   endPerformanceTimer('sassErrorPage', timer)
 }