Avoid overwriting the text templates if they already exist

[philderbeast]

Found this when using Updo in a Github action run when stackage was setup locally but because of the recipe overwrite, a constraint conflict arose pulling in Stackage's cabal.config:

$ cabal build all --enable-tests --enable-benchmarks --dry-run
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: hpack-dhall-0.5.7 (user goal)
[__1] next goal: hpack (dependency of hpack-dhall)
[__1] rejecting: hpack-0.36.0 (constraint from project config
https://stackage.org/lts-21.25/cabal.config requires ==0.35.2)

Here's how to reproduce it using touch, when project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall and project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall already exist.

$ touch updo/project-skeleton/ghc-x.y.z/text-templates/dhall2cabal.dhall

$ make -f project-files.mk cabal.project
mkdir -p project-dhall/pkgs
[ -e project-dhall/pkgs/hpack-dhall.dhall ] || cp updo/project-skeleton/pkgs/all-in-one.dhall project-dhall/pkgs/all-in-one.dhall
mkdir -p project-dhall
[ -e project-dhall/pkg-groups.dhall ] || cp updo/project-skeleton/pkg-groups.dhall project-dhall/pkg-groups.dhall
mkdir -p project-dhall/ghc-9.4.8/text-templates
cp updo/project-skeleton/ghc-x.y.z/text-templates/cabal-snippet.dhall project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall
cp updo/project-skeleton/ghc-x.y.z/text-templates/dhall2cabal.dhall project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall
echo './project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall ./.updo/pkgs-sorted.dhall "lts-21.25"' \
| dhall text | sed -E "s/@(rev|sha256):.*$//" > ghc-9.4.8.dhall2cabal.project
cp ghc-9.4.8.dhall2cabal.project cabal.project
rm ghc-9.4.8.dhall2cabal.project

The overwriting changes the configured set up:

$ git diff
diff --git a/cabal.project b/cabal.project
old mode 100755
new mode 100644
index 81dda4f..7c869ca
--- a/cabal.project
+++ b/cabal.project
@@ -1,8 +1,10 @@
-import: ./project-stackage/lts-21.25.config
+import: https://stackage.org/lts-21.25/cabal.config
 
+-- We have 1 package.
 packages:
     .
 
+-- Version equality constraints.
 constraints:
     crypton ==1.0.0
   , crypton-connection ==0.4.0
@@ -13,6 +15,6 @@ constraints:
   , hpack ==0.36.0
   , http-client-tls ==0.3.6.3
   , tls ==2.0.5
-
 program-options
   ghc-options: -fhide-source-paths
+
diff --git a/project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall b/project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall
index d8ba517..2d7a4ee 100644
--- a/project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall
+++ b/project-dhall/ghc-9.4.8/text-templates/cabal-snippet.dhall
@@ -1,4 +1,4 @@
 ''
-
 program-options
-  ghc-options: -fhide-source-paths''
+  ghc-options: -fhide-source-paths
+''
diff --git a/project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall b/project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall
index 49ca975..fc7b24d 100644
--- a/project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall
+++ b/project-dhall/ghc-9.4.8/text-templates/dhall2cabal.dhall
@@ -22,8 +22,8 @@ in  \(pkgs-done : List Text) ->
 
       in  ''
           ${../../../updo/text-templates/dhall2cabal.dhall
-              TYPES.Verbosity.Quiet
-              TYPES.Stackage.StackageLocal
+              TYPES.Verbosity.Info
+              TYPES.Stackage.StackageWeb
               stackage-resolver
               ( if    null Text pkgs-todo
                 then  TYPES.PkgSet.AllPkgs pkgs-done