This is needed to handle compound filetypes since we cant imply the
location of the syntax checker file from the filetype.
e.g. we want to load `syntax_checkers/python/pylint.vim`, but the
filetype is `python.django`. Previously this was causing `runtime
syntax_checkers/python.django/pylint.vim` to be executed.
Javascript and json have multiple syntax checkers that can be loaded.
Previously the logic to determine which checker to load was basically
copied and pasted in both. The `go` checker will soon have more
than one option too so remove the duplication by sticking the
common code in the core.
