Handle all pip install flags properly while generating requirements.txt

With this patch, the plugin will respect all pip install short flags,
not just -f and -i.
https://pip.pypa.io/en/stable/reference/pip_install/#requirements-file-format

Author: jacksgt

lib/pip.js

@@ -372,7 +372,7 @@ function getRequirements(source) {
  *  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 ) or any
- *  lines that start with -f or -i,  Please see:
+ *  lines that start with -c, -e, -f, -i or -r,  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
@@ -389,8 +389,11 @@ function filterRequirementsFile(source, target, options) {
       return false;
     } else if (
       req.startsWith('--') ||
+      req.startsWith('-c') ||
+      req.startsWith('-e') ||
       req.startsWith('-f') ||
-      req.startsWith('-i')
+      req.startsWith('-i') ||
+      req.startsWith('-r')
     ) {
       // If we have options (prefixed with --) keep them for later
       prepend.push(req);