PY-31773 Prepend (not append) virtualenv's bin/ directory to PATH

It should take precedence according to the virtualenvs 'activate' script.

python/src/com/jetbrains/python/run/PythonCommandLineState.java

@@ -383,7 +383,7 @@ public abstract class PythonCommandLineState extends CommandLineState {
       for (Map.Entry<String, String> e : myConfig.getEnvs().entrySet()) {
         if (environment.containsKey(e.getKey())) {
           if ("PATH".equals(e.getKey())) {
-            env.put(e.getKey(), PythonEnvUtil.appendToPathEnvVar(env.get("PATH"), e.getValue()));
+            env.put(e.getKey(), PythonEnvUtil.addToPathEnvVar(env.get("PATH"), e.getValue(), true));
           }
           else {
             env.put(e.getKey(), e.getValue());

python/src/com/jetbrains/python/sdk/PythonEnvUtil.java

@@ -60,18 +60,29 @@ public class PythonEnvUtil {
   }
 
   /**
-   * Appends a value to the end os a path-like environment variable, using system-dependent path separator.
+   * Adds a value to the end os a path-like environment variable, using system-dependent path separator.
    *
    * @param source path-like string to append to
    * @param value  what to append
+   * @param asPrefix if true, adds a value as a prefix, otherwise as a suffix
    * @return modified path-like string
    */
 
   @NotNull
-  public static String appendToPathEnvVar(@Nullable String source, @NotNull String value) {
+  public static String addToPathEnvVar(@Nullable String source, @NotNull String value, boolean asPrefix) {
     if (StringUtil.isEmpty(source)) return value;
     Set<String> paths = Sets.newHashSet(source.split(File.pathSeparator));
-    return !paths.contains(value) ? source + File.pathSeparator + value : source;
+    if (!paths.contains(value)) {
+      if (asPrefix) {
+        return value + File.pathSeparator + source;
+      }
+      else {
+        return source + File.pathSeparator + value;
+      }
+    }
+    else {
+      return source;
+    }
   }
 
   public static void addPathsToEnv(@NotNull Map<String, String> env, String key, @NotNull Collection<String> values) {
@@ -83,7 +94,7 @@ public class PythonEnvUtil {
   public static void addPathToEnv(@NotNull Map<String, String> env, String key, String value) {
     if (!StringUtil.isEmpty(value)) {
       if (env.containsKey(key)) {
-        env.put(key, appendToPathEnvVar(env.get(key), value));
+        env.put(key, addToPathEnvVar(env.get(key), value, false));
       }
       else {
         env.put(key, value);

python/src/com/jetbrains/python/sdk/PythonSdkType.java

@@ -363,11 +363,11 @@ public final class PythonSdkType extends SdkType {
       Map<String, String> env = commandLine.getEnvironment();
       String pathValue;
       if (env.containsKey(PATH)) {
-        pathValue = PythonEnvUtil.appendToPathEnvVar(env.get(PATH), virtualenvBin);
+        pathValue = PythonEnvUtil.addToPathEnvVar(env.get(PATH), virtualenvBin, true);
       }
       else if (passParentEnvironment) {
         // append to PATH
-        pathValue = PythonEnvUtil.appendToPathEnvVar(System.getenv(PATH), virtualenvBin);
+        pathValue = PythonEnvUtil.addToPathEnvVar(System.getenv(PATH), virtualenvBin, true);
       }
       else {
         pathValue = virtualenvBin;