Fix linux build

[finestructure]

On Linux, the @main struct cannot be in a library target that is then imported by the executable target. This works on macOS but raises the following build error on Linux:

error: link command failed with exit code 1 (use -v to see invocation)
/usr/bin/ld.gold: error: undefined symbol 'pg_extras_main' referenced in expression
clang-13: error: linker command failed with exit code 1 (use -v to see invocation)

This work-around moves the top level command into the executable target.