Rename libshortfin -> shortfin.

[stellaraccident]

Now that the old POC directory is out of the way, we can slide this to its proper place.

I left a few references to libshortfin in the codebase:

• Places where we are referring to the C++ library (literally libshortfin).
• GitHub YAML file names (those can be annoying to change and are better done in a standalone PR).
• CMake project name (can have things that depend on this name).