Rename shell executor files

Overview

At the moment we have the word executor_ when we are already under the executors/shell directory which seems a bit redundant. Let's rename the files:

Right now:

$ ls -la executors/shell
drwxr-xr-x   5 steve  staff    160 Mar  2 09:27 ./
drwxr-xr-x  12 steve  staff    384 Feb 26 17:36 ../
-rw-r--r--   1 steve  staff   3652 Mar  2 09:27 executor_shell.go
-rw-r--r--   1 steve  staff  38582 Feb 27 08:27 executor_shell_integration_test.go
-rw-r--r--   1 steve  staff   1033 Mar  2 09:27 executor_shell_terminal.go

Ideally:

$ ls -la executors/shell
drwxr-xr-x   5 steve  staff    160 Mar  2 09:27 ./
drwxr-xr-x  12 steve  staff    384 Feb 26 17:36 ../
-rw-r--r--   1 steve  staff   3652 Mar  2 09:27 shell.go
-rw-r--r--   1 steve  staff  38582 Feb 27 08:27 shell_integration_test.go
-rw-r--r--   1 steve  staff   1033 Mar  2 09:27 shell_terminal.go