Compilers-as-nodes discussion

[AlexanderRichert-NOAA]

Since 5b3942a48935ee4aeef724b4a28c9666a75821c4, Spack implements compilers as nodes which are providers of languages ("provides('c')", "provides('fortran')", etc.). This will involve many changes to how we set compilers, and needs extensive testing as it is still a work in progress.

Let's use this issue to record issues encountered, best practices, tricks, etc.