fix: don't rely on string context for package source

Before this commit, the type of a package source (haskell-source-type)
depended on string contexts in its `check` function. However, string
contexts do not work well to represent a path to a package source. For
instance, "hello ./subdirectory" would have a string context, but is
obviously not a valid path.

Furthermore, a `nix` bug (NixOS/nix#8428)
related to string contexts triggers an issue where setting the project
root causes a type error with haskell-source-type (srid#169).

This commit replaces haskell-source-type with `with type; either path str`
and instances of `isPathUnderNixStore` with `lib.types.path.check` to
avoid using string contexts.

nix/modules/project/packages/default.nix

@@ -55,12 +55,11 @@ in
       default = self: _super:
         let
           inherit (project.config) log;
-          isPathUnderNixStore = path: builtins.hasContext (builtins.toString path);
           build-haskell-package = import ../../../build-haskell-package.nix {
             inherit pkgs lib self log;
           };
           getOrMkPackage = name: cfg:
-            if isPathUnderNixStore cfg.source
+            if lib.types.path.check cfg.source
             then
               log.traceDebug "${name}.callCabal2nix ${cfg.source}"
                 (build-haskell-package name cfg.source)

nix/modules/project/packages/package.nix

@@ -4,19 +4,17 @@ let
   inherit (lib)
     mkOption
     types;
-  # TODO: DRY
-  isPathUnderNixStore = path: builtins.hasContext (builtins.toString path);
 
   # Whether the 'path' is local to `project.config.projectRoot`
   localToProject = path:
     path != null &&
-    isPathUnderNixStore path &&
+    lib.types.path.check path &&
     lib.strings.hasPrefix "${project.config.projectRoot}" "${path}";
 in
 { name, config, ... }: {
   options = {
     source = mkOption {
-      type = import ../../../types/haskell-source-type.nix { inherit lib; };
+      type = with types; either path str;
       description = ''
         Source refers to a Haskell package defined by one of the following:
 
@@ -43,7 +41,7 @@ in
             '';
           };
         in
-        if isPathUnderNixStore config.source
+        if lib.types.path.check config.source
         then haskell-parsers.getCabalExecutables config.source
         else null; # cfg.source is Hackage version; nothing to do.
     };