My Raspberry Pi is In a boot loop, but I know which files (the motion package) is corrupting the system, so my question is: How do I boot into a command line on Raspbian?
Put the SD Card from your Raspi into the Card Reader of your PC and edit
cmdline.txt. Append to the command line:
A special target unit that starts an emergency shell on the main console. This target does not pull in any services or mounts. It is the most minimal version of starting the system in order to acquire an interactive shell 
You can also try to use:
It's less restrict.
A special target unit that pulls in the base system (including system mounts) and spawns a rescue shell. Isolate to this target in order to administer the system in single-user mode with all file systems mounted but with no services running, except for the most basic .
Then boot your raspi with the SD Card.
 Special System Units