module: improve error for invalid package targets

For targets that are strings that do not start with `./` or `/` the
error will now have additional information about what the programming
error is.

Closes: #32034

lib/internal/errors.js

@@ -1104,18 +1104,28 @@ E('ERR_INVALID_PACKAGE_CONFIG', (path, message, hasMessage = true) => {
 }, Error);
 E('ERR_INVALID_PACKAGE_TARGET',
   (pkgPath, key, subpath, target, base = undefined) => {
+    const relError = typeof target === 'string'
+      && target.length && !target.startsWith('./');
     if (key === null) {
       if (subpath !== '') {
         return `Invalid "exports" target ${JSONStringify(target)} defined ` +
         `for '${subpath}' in the package config ${pkgPath} imported from ` +
-        base;
+        `${base}.${relError ? ' - targets must start with "./"' : ''}`;
       } else {
         return `Invalid "exports" main target ${target} defined in the ` +
-          `package config ${pkgPath} imported from ${base}.`;
+          `package config ${pkgPath} imported from ${base}.${relError ?
+          ' - targets must start with "./"' : ''}`;
       }
     } else if (key === '.') {
       return `Invalid "exports" main target ${JSONStringify(target)} defined ` +
-      `in the package config ${pkgPath}${sep}package.json`;
+      `in the package config ${pkgPath}${sep}package.json${relError ?
+            ' - targets must start with "./"' : ''}`;
+    } else if (typeof target === 'string' && target !== '' &&
+      !target.startsWith('./')) {
+        return `Invalid "exports" target ${JSONStringify(target)} defined for '${
+          StringPrototypeSlice(key, 0, -subpath.length || key.length)}' in the ` +
+          `package config ${pkgPath}${sep}package.json. ` +
+          '- targets must start with `./`';
     } else {
       return `Invalid "exports" target ${JSONStringify(target)} defined for '${
         StringPrototypeSlice(key, 0, -subpath.length || key.length)}' in the ` +

test/es-module/test-esm-exports.mjs

@@ -78,6 +78,7 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
     ['pkgexports/null', './null'],
     ['pkgexports/invalid2', './invalid2'],
     ['pkgexports/invalid3', './invalid3'],
+    ['pkgexports/invalid5', 'invalid5'],
     // Missing / invalid fallbacks
     ['pkgexports/nofallback1', './nofallback1'],
     ['pkgexports/nofallback2', './nofallback2'],
@@ -106,6 +107,9 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
       strictEqual(err.code, 'ERR_INVALID_PACKAGE_TARGET');
       assertStartsWith(err.message, 'Invalid "exports"');
       assertIncludes(err.message, subpath);
+      if (!subpath.startsWith('./')) {
+        assertIncludes(err.message, 'targets must start with');
+      }
     }));
   }