scancode does not properly handle multiple explicit paths

### Description

> Please leave a brief description of the bug or feature request:

I am trying to use scancode to detect multiple license files within a directory. I expect the tool to list multiple `"type": "file"` entries in the output JSON file with the `path`s relative to the current directory. Instead, I get a bunch of directory entries with no detected license files. It works as expected when I pass a single license file.

### How To Reproduce

> Tell us how to reproduce the issue.

In an unpacked moby/moby git source archive (but really any source tree with multiple licenses works), I ran:

``` shell
scancode --json-pp abcd.json --license ./vendor/github.com/inconshreveable/mousetrap/LICENSE ./vendor/github.com/google/shlex/COPYING
```

### System configuration

> For bug reports, it really helps us to know:

* What OS are you running on? (Windows/MacOS/Linux) Linux

* What version of scancode-toolkit was used to generate the scan file?

``` console
$ scancode --version
ScanCode version: 32.1.0
ScanCode Output Format version: 3.1.0
SPDX License list version: 3.23
```

* What installation method was used to install/run scancode? (pip/source download/other) pipx