Add "software collections" to kit scanning on Linux

RedHat and CentOS systems use "software collections" to make new compilers available:

https://www.softwarecollections.org/en/scls/rhscl/devtoolset-8/

These compilers live in predictable locations, and cmake-tools kit scanning should check these locations for kit choices, perhaps even labelled "devtoolset-8", etc.

The issue names cmake-tools kit scanning but no source files or tests. Start by locating the Linux kit-scanning entry point and existing compiler discovery logic; done means predictable Red Hat and CentOS software-collection compiler locations are scanned and the resulting kits can be labeled with names such as devtoolset-8.

Written by the indexing model from the issue text.