Handle wildcards in package definitions

When the user specifies

```bash
"/software/packages" = pkg_repl("python*", "1.2.3", "x86_64");
```

he'll lock down all packages that `python`, and have version "1.2.3".

So, specifying

```bash
"/software/packages/{python-devel}" = nlist();
```

will bring a version of `python-devel` that is consistent with the locked Python.

Only one issue needs to be addressed: if the user locks a string that doesn't match a package name (say, "pyth"), SPMA may fail because it won't be able to install package "pyth".  What should we do in that case?