oss-licenses-plugin does not pick up licenses from parent POMs, so some popular libraries (e.g. Apache Commons) don't have license info displayed

I have a dependency on ZXing, which stores the license information in the parent POM and then cascades this down to child POMs.

Here's the latest (at time of writing) version of the child POM: https://repo1.maven.org/maven2/com/google/zxing/core/3.4.1/core-3.4.1.pom

Example Gradle dependency:

implementation 'com.google.zxing:core:3.4.1'

The license information is not shown in the generated `third_party_licenses` data (nor in the activity for the play services library).

I suspect this is because it stores the information through the parent POM:

```

com.google.zxing
zxing-parent
3.4.1

```

and then in [the parent](https://repo1.maven.org/maven2/com/google/zxing/zxing-parent/3.4.1/zxing-parent-3.4.1.pom):

```
com.google.zxing
zxing-parent
3.4.1
pom
...

The Apache Software License, Version 2.0
https://www.apache.org/licenses/LICENSE-2.0.txt
repo

```

The [MVNRepository entry](https://mvnrepository.com/artifact/com.google.zxing/core/3.4.1) is able to show the license information even though it's in the parent POM; is it possible for oss-licenses-plugin to support this too?