Scanning multiple directories scans too much

### Description

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

### How To Reproduce

> Tell us how to reproduce the issue.

We have a giant third_party/ directory. GIANT! Trying to scan one package works fine. But trying to scan two at once, it scans things outside of those directories

``` bash
$ scancode -n 129 --copyright --license --package --json /tmp/out.json --max-in-memory 0 third_party/curl third_party/zlib
Setup plugins...
Collect file inventory...

```

It appears to hang there, but strace shows that it's actually scanning things *outside* of the curl and zlib directories, which will take a long time.

### System configuration

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

* What OS are you running on? (Windows/MacOS/Linux): Debian testing based system.
* What version of scancode-toolkit was used to generate the scan file? 32.0.4
* What installation method was used to install/run scancode? (pip/source download/other)