Fix go.mod parser for repeated replace directives

Constructs such as
```
replace google.golang.org/grpc => google.golang.org/grpc v1.33.2
replace github.com/gogo/protobuf => github.com/regen-network/protobuf v1.3.3-alpha.regen.1
```

Are perfectly valid and we previously discarded every one but the last of them.

builder/parser.nix

@@ -3,7 +3,7 @@
 # in normalised form.
 
 let
-  inherit (builtins) elemAt mapAttrs split foldl' match filter typeOf;
+  inherit (builtins) elemAt mapAttrs split foldl' match filter typeOf hasAttr length;
 
   # Strip lines with comments & other junk
   stripStr = s: elemAt (split "^ *" (elemAt (split " *$" s) 0)) 2;
@@ -16,9 +16,6 @@ let
     # Strip leading tabs characters
     (lines: map (l: elemAt (match "(\t)?(.*)" l) 1) lines)
 
-    # Strip comment lines
-    (filter (l: match "[ \t]*//.*" l != null))
-
     # Filter empty lines
     (filter (l: l != ""))
   ];
@@ -40,21 +37,40 @@ let
 
       in
       {
-        data = acc.data // (
+        data = (acc.data // (
           if directive == "" && rest == ")" then { }
-          else if inDirective != null && rest == "(" then {
+          else if inDirective != null && rest == "(" && ! hasAttr inDirective acc.data then {
             ${inDirective} = { };
-          } else if inDirective != null then {
+          }
+          else if rest == "(" || rest == ")" then { }
+          else if inDirective != null then {
             ${inDirective} = acc.data.${inDirective} // { ${directive} = rest; };
-          } else {
+          } else if directive == "replace" then
+            (
+              let
+                segments = split " => " rest;
+                getSegment = elemAt segments;
+              in
+              assert length segments == 3; {
+                replace = acc.data.replace // {
+                  ${getSegment 0} = "=> ${getSegment 2}";
+                };
+              }
+            )
+          else {
             ${directive} = rest;
           }
+        )
         );
         inherit inDirective;
       })
     {
       inDirective = null;
-      data = { };
+      data = {
+        require = { };
+        replace = { };
+        exclude = { };
+      };
     }
     lines
   ).data;
@@ -98,14 +114,14 @@ let
     data // {
       replace =
         mapAttrs
-          (n: v:
+          (_: v:
             let
               m = match "=> ([^ ]+) (.+)" v;
               m2 = match "=> (.*+)" v;
             in
             if m != null then {
               goPackagePath = elemAt m 0;
-              version = parseVersion (elemAt m 1);
+              version = elemAt m 1;
             } else {
               path = elemAt m2 0;
             })
@@ -113,12 +129,6 @@ let
     }
   );
 
-  parseRequire = data: (
-    data // {
-      require = mapAttrs (n: v: parseVersion v) data.require;
-    }
-  );
-
   splitString = sep: s: filter (t: t != [ ]) (split sep s);
 
 in
@@ -128,5 +138,4 @@ foldl' (acc: f: f acc) (splitString "\n" contents) [
   parseLines
   normaliseDirectives
   parseReplace
-  parseRequire
 ]