fix(paths/packages): treat only the literal basename as a manifest

`resolvePackageJsonDirname` and `resolvePackageJsonPath` used
`filepath.endsWith('package.json')`, which matched any filename whose
suffix happened to be `package.json` — e.g. `/foo/my-package.json`. The
first would strip it to `/foo`; the second would keep it verbatim and
skip the append. Now both gate on `path === 'package.json'` or a literal
`/package.json`/`\package.json` suffix, so only real manifests are
classified as manifests regardless of the host separator. Adds regression
tests for `my-package.json` under both APIs.

Author: jdalton

src/paths/packages.ts

@@ -18,12 +18,26 @@ function getPath() {
   return _path as typeof import('node:path')
 }
 
+/**
+ * Whether `filepath`'s final segment is exactly `package.json`. Accepts both
+ * POSIX and Windows-style separators so paths captured on either platform
+ * classify the same regardless of the host we're running on.
+ */
+/*@__NO_SIDE_EFFECTS__*/
+function isPackageJsonFile(filepath: string): boolean {
+  return (
+    filepath === 'package.json' ||
+    filepath.endsWith('/package.json') ||
+    filepath.endsWith('\\package.json')
+  )
+}
+
 /**
  * Resolve directory path from a package.json file path.
  */
 /*@__NO_SIDE_EFFECTS__*/
 export function resolvePackageJsonDirname(filepath: string): string {
-  if (filepath.endsWith('package.json')) {
+  if (isPackageJsonFile(filepath)) {
     const path = getPath()
     return normalizePath(path.dirname(filepath))
   }
@@ -35,7 +49,7 @@ export function resolvePackageJsonDirname(filepath: string): string {
  */
 /*@__NO_SIDE_EFFECTS__*/
 export function resolvePackageJsonPath(filepath: string): string {
-  if (filepath.endsWith('package.json')) {
+  if (isPackageJsonFile(filepath)) {
     return normalizePath(filepath)
   }
   const path = getPath()

test/unit/paths/packages.test.mts

@@ -53,6 +53,17 @@ describe('paths/packages', () => {
       const result = resolvePackageJsonDirname('/foo/bar/baz/qux/package.json')
       expect(result).toBe('/foo/bar/baz/qux')
     })
+
+    it('should not strip to parent for files merely ending in package.json', () => {
+      // Regression: `endsWith('package.json')` used to misclassify
+      // `my-package.json` as a manifest and return the parent dir.
+      expect(resolvePackageJsonDirname('/foo/my-package.json')).toBe(
+        '/foo/my-package.json',
+      )
+      expect(resolvePackageJsonDirname('/foo/bar-package.json')).toBe(
+        '/foo/bar-package.json',
+      )
+    })
   })
 
   describe('resolvePackageJsonPath', () => {
@@ -93,6 +104,14 @@ describe('paths/packages', () => {
       expect(result).toBe('/foo/bar/package.json')
       expect(result).not.toBe('/foo/bar/package.json/package.json')
     })
+
+    it('should append package.json to files that merely end in package.json', () => {
+      // Regression: `endsWith('package.json')` used to skip the append
+      // for paths like `/foo/my-package.json`, treating them as manifests.
+      expect(resolvePackageJsonPath('/foo/my-package.json')).toBe(
+        '/foo/my-package.json/package.json',
+      )
+    })
   })
 
   describe('integration', () => {