modules/npm: Make the generated workspace package private with a stable name

The autogenerated packages/hugoautogen/package.json got its name from the
project directory basename and had no "private" field, making it publishable
under checkout-dependent names. Use the stable default name "hugoautogen",
set "private": true, and preserve hand-set name/version/private on
regeneration.

Fixes #15245

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

testscripts/commands/mod_npm.txt

@@ -119,7 +119,8 @@ usePackageJSON="auto"
     "strip-ansi": "7.2.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden1/package.json --
@@ -149,7 +150,8 @@ usePackageJSON="auto"
     "strip-ansi": "7.2.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden3/packages/hugoautogen/package.json --
@@ -168,7 +170,8 @@ usePackageJSON="auto"
     "strip-ansi": "7.2.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden4/packages/hugoautogen/package.json --
@@ -188,7 +191,8 @@ usePackageJSON="auto"
     "strip-ansi": "7.2.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden5/packages/hugoautogen/package.json --
@@ -206,7 +210,8 @@ usePackageJSON="auto"
     "strip-ansi": "7.2.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- go.mod --

testscripts/commands/mod_npm__moduleorder.txt

@@ -42,7 +42,8 @@ go 1.20
     "strip-ansi": "7.0.0",
     "to-pascal-case": "1.0.0"
   },
-  "name": "script-mod_npm__moduleorder",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden1/packages/hugoautogen/hugo_packagemeta.json --

testscripts/commands/mod_npm_withexisting.txt

@@ -37,7 +37,8 @@ path="github.com/gohugoio/hugoTestModule2"
     "@babel/preset-env": "7.9.5",
     "postcss-cli": "7.1.0"
   },
-  "name": "script-mod_npm_withexisting",
+  "name": "hugoautogen",
+  "private": true,
   "version": "0.1.0"
 }
 -- golden/package.json --