Path mappings have unintuitive behavior if user accidentally adds an extension

[DanielRosenwasser]

Was with @zhengbli when we hit the following:

tsconfig.json:

{
    "compilerOptions": {
        "baseUrl": ".",
    	"paths": {
             "foo": ["foo.ts"]
        }
    }
}

foo.ts

export function blah() { }

hello.ts:

import * as x from "foo";

Right now you'll get an error on the import that foo couldn't be resolved.

Even the documentation currently states (incorrectly) that this should be a valid (though the website isn't up to date). Perhaps we should allow users to specify the extension.

[vladima]

it used to work, seems it was broken in this PR. @Andy-MS, can you please take a look