feature: make several packages detectable

[upsj]

This makes a handful of common packages detectable by adding their libraries/executable names. I checked with apt that the naming scheme of the DSOs matches the version numbers, as I had to exclude a few other libraries (libffi, libpciaccess, ...) for a differing ABI versioning scheme. For those, I guess one would have to parse the config headers.

[spackbot-app]

Hi @upsj! I noticed that the following package(s) don't yet have maintainers:

• libbsd
• libxml2
• pigz
• re2c
• readline
• source-highlight
• zlib

Are you interested in adopting any of these package(s)? If so, simply add the following to the package class:

    maintainers = ["upsj"]

If not, could you contact the developers of this package and see if they are interested? You can quickly see who has worked on a package with spack blame:

$ spack blame libbsd

Thank you for your help! Please don't add maintainers without their consent.

You don't have to be a Spack expert or package developer in order to be a "maintainer," it just gives us a list of users willing to review PRs or debug issues relating to this package. A package can have multiple maintainers; just add a list of GitHub handles of anyone who wants to volunteer.

[haampie]

I appreciate having more packages support external find, but the main caveat is that we have to ensure the <pkg>-dev system package is installed, not just the runtime <pkg> package.

So, basically, detect libsomething.<so-extension> without trailing ABI version .x or API version .a.b.c.

Also, what about macOS? Hard-coded .so isn't very helpful there.