fix: resolve modules if folder contains a package.json file

getMappedPath only resolves to files that exist. This does not
work when a TSConfig path alias resolves to a module folder
containing built assets, and described by a package.json, e.g.
foo/dist.

If the tsconfig path is mapped to a folder, then getMappedPath
returns undefined as the folder is not a file. However, in somes
cases it is ok for this to resolve as the package.json contains
a `typings` entry that references a .d.ts file, which is
correctly handled by enhanced-resolve

src/index.ts

@@ -250,6 +250,10 @@ const isFile = (path?: string | undefined): path is string => {
   }
 }
 
+const isModule = (path?: string | undefined): path is string => {
+  return !!path && isFile(`${path}/package.json`)
+}
+
 /**
  * @param {string} source the module to resolve; i.e './some-module'
  * @param {string} file the importing file's full path; i.e. '/usr/local/bin/file.js'
@@ -267,7 +271,7 @@ function getMappedPath(
   const originalExtensions = extensions
   extensions = ['', ...extensions]
 
-  let paths: string[] | undefined = []
+  let paths: Array<string | undefined> | undefined = []
 
   if (RELATIVE_PATH_PATTERN.test(source)) {
     const resolved = path.resolve(path.dirname(file), source)
@@ -283,7 +287,7 @@ function getMappedPath(
         ]),
       )
       .flat(2)
-      .filter(isFile)
+      .filter(path => isFile(path) || isModule(path))
   }
 
   if (retry && paths.length === 0) {

tests/withPaths/index.ts

@@ -10,6 +10,9 @@ import 'folder/tsxImportee'
 import 'folder/subfolder/tsImportee'
 import 'folder/subfolder/tsxImportee'
 
+// import module with typings set in package.json
+import 'folder/module'
+
 // import from node_module
 import 'typescript'
 import 'dummy.js'

tests/withPaths/module/module.d.ts

@@ -0,0 +1 @@
+export {}

tests/withPaths/module/package.json

@@ -0,0 +1,5 @@
+{
+  "type": "commonjs",
+  "typings": "./module.d.ts",
+  "private": true
+}