Feature: generating pkg-config files

Now that we have support for libraries, it would be interesting to add support for generating pkg-config `.pc` files. These are text configuration files which provide a way for buildsystems to check what flags are required to be able to find headers and libraries for a C library. Here's a sample of what one looks like:

```
prefix=/opt/homebrew/Cellar/harfbuzz/8.5.0
includedir=${prefix}/include
libdir=${prefix}/lib

Name: harfbuzz
Description: HarfBuzz text shaping library
Version: 8.5.0
Requires.private: freetype2 >= 12.0.6, glib-2.0 >= 2.30.0, graphite2 >= 1.2.0
Libs: -L${libdir} -lharfbuzz
Libs.private: -lm -framework ApplicationServices
Cflags: -I${includedir}/harfbuzz
```

A few things we'll want to consider:

* Do we want all libraries in a package to get one `.pc` file, or one per library? (This will probably want to be configurable.)
* Will we use the package name for the `.pc` file, or make it configurable?

cc https://github.com/axodotdev/cargo-dist/issues/179#issuecomment-2249517786

The issue names no source files, tests, or implementation entry points. Start by reviewing the existing library packaging support and the linked cargo-dist discussion, then resolve whether packages produce one .pc file or one per library and whether the filename is configurable. Done means generated pkg-config files contain the required library metadata, flags, and dependencies.

Written by the indexing model from the issue text.