Adding -f and -i to requirements.txt options ignore list before sorting (#238)

Fixes: #236 (again, properly)

Problem: requirements.txt options which must be in the proper order at the start of the file were being sorted.  This properly skips sorting them and keeps them in order they were.

Author: AndrewFarley

lib/pip.js

@@ -281,7 +281,9 @@ function dockerPathForWin(options, path) {
  *  then remove all comments and empty lines, and sort the list which
  *  assist with matching the static cache.  The sorting will skip any
  *  lines starting with -- as those are typically ordered at the
- *  start of a file ( eg: --index-url / --extra-index-url )
+ *  start of a file ( eg: --index-url / --extra-index-url ) or any
+ *  lines that start with -f or -i,  Please see:
+ * https://pip.pypa.io/en/stable/reference/pip_install/#requirements-file-format
  * @param {string} source requirements
  * @param {string} target requirements where results are written
  * @param {Object} options
@@ -297,7 +299,11 @@ function generateRequirementsFile(source, target, options) {
     if (req.startsWith('#')) {
       // Skip comments
       return false;
-    } else if (req.startsWith('--')) {
+    } else if (
+      req.startsWith('--') ||
+      req.startsWith('-f') ||
+      req.startsWith('-i')
+    ) {
       // If we have options (prefixed with --) keep them for later
       prepend.push(req);
       return false;
@@ -309,7 +315,9 @@ function generateRequirementsFile(source, target, options) {
   filteredRequirements.sort(); // Sort remaining alphabetically
   // Then prepend any options from above in the same order
   for (let item of prepend.reverse()) {
-    filteredRequirements.unshift(item);
+    if (item && item.length > 0) {
+      filteredRequirements.unshift(item);
+    }
   }
   fse.writeFileSync(target, filteredRequirements.join('\n') + '\n');
 }