Mine pkg-config files in source code packages

Some packages have a so called "pkg-config" files. It is specified in `man 5 pc` on many recent Linux systems. There are some fields in it that could be useful to help detect provenance, such as the `URL` field. An example:

```
Name: bash-completion
Description: programmable completion for the bash shell
URL: https://github.com/scop/bash-completion
Version: 2.11
prefix=/usr
includedir=${prefix}/include
```

Although `URL` is mandatory to the specification, many pkg-config files do not have it.