fix: don't split :<word> into two tokens.

In the situation where we have `:im<tab>` normally `:` and `im` would be split into
two tokens and it would result in `::imports`. Instead, we detect early if we are
about to complete a command and instead of parsing it normally like Scala we just
grab the entire thing as the word.

compiler/src/dotty/tools/repl/JLineTerminal.scala

@@ -152,6 +152,14 @@ final class JLineTerminal extends java.io.Closeable {
           // using dummy values, resulting parsed input is probably unused
           defaultParsedLine
 
+        // In the situation where we have a partial command that we want to
+        // complete we need to ensure that the :<partial-word> isn't split into
+        // 2 tokens, but rather the entire thing is treated as the "word", in
+        //   order to insure the : is replaced in the completion.
+        case ParseContext.COMPLETE if
+          ParseResult.commands.exists(command => command._1.startsWith(input)) =>
+            parsedLine(input, cursor)
+
         case ParseContext.COMPLETE =>
           // Parse to find completions (typically after a Tab).
           def isCompletable(token: Token) = isIdentifier(token) || isKeyword(token)

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -221,13 +221,7 @@ class ReplDriver(settings: Array[String],
 
     if expr.startsWith(":") then
       ParseResult.commands.collect {
-        // If expr is only : then we return the commands with : since jline
-        // correctly handles them
-        case command if expr == ":" => makeCandidate(command._1)
-        // However if there is more than just the : we filter by it and then
-        // drop the : since jline will correctly complete it but you'll end up
-        // with ::import for example instead of :import
-        case command if command._1.startsWith(expr) => makeCandidate(command._1.drop(1))
+        case command if command._1.startsWith(expr) => makeCandidate(command._1)
       }
     else
       given state: State = newRun(state0)

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -211,7 +211,7 @@ class TabcompleteTests extends ReplTest {
   @Test def commandPreface = initially {
     // This looks odd, but if we return :doc here it will result in ::doc in the REPL
     assertEquals(
-      List("doc"),
+      List(":doc"),
       tabComplete(":d")
     )
   }