mesa: support Meson installation tags

This MR adds support for Meson's installation tags feature, which allows splitting installed files into separate libfoo, libfoo-dev, etc. packages. I thought this would be a simple enough change, and packagers will probably appreciate it :)

Meson already tags most of the files automatically, so the feature worked out of the box in other Mesa repos. However this one also has custom install targets, which must be tagged manually. I've separated each kind into separate commits to ease review, but most importantly these are ICDs, plus the custom install script for megadrivers. /cc @eric

It's the first time I submit a patch here, I'm not sure if you'd prefer me to squash into a single commit (with the title / description of the MR as commit message)?