Refactor `BASH_SOURCE` usage

We now assign once at the top; this makes portability changes with
regard to this variable easier.

activate.sh

 #!/bin/bash
 
-RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+SOURCE_PATH="${BASH_SOURCE[0]}"
+
+RELATIVE_PATH="$(dirname "$SOURCE_PATH")"
 ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 
-[[ "$0" != "${BASH_SOURCE[0]}" ]] && echo "The activation script must be sourced, otherwise the virtual environment will not work." || ( echo "Vars script must be sourced." && exit 1) ;
+[[ "$0" != "${SOURCE_PATH}" ]] && echo "The activation script must be sourced, otherwise the virtual environment will not work." || ( echo "Vars script must be sourced." && exit 1) ;
 
 source "${ABSOLUTE_PATH}"/config.sh
 source "${ABSOLUTE_PATH}"/modules.sh

config.sh

+SOURCE_PATH="${BASH_SOURCE[0]}"
+
 ## Check if this script is sourced
-[[ "$0" != "${BASH_SOURCE[0]}" ]] && echo "Setting vars" || ( echo "Vars script must be sourced." && exit 1) ;
+[[ "$0" != "${SOURCE_PATH}" ]] && echo "Setting vars" || ( echo "Vars script must be sourced." && exit 1) ;
 ## Determine location of this file
-RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+RELATIVE_PATH="$(dirname "$SOURCE_PATH")"
 ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 ####################################
 

create_kernel.sh

 #!/bin/bash
 
-RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+SOURCE_PATH="${BASH_SOURCE[0]}"
+
+RELATIVE_PATH="$(dirname "$SOURCE_PATH")"
 ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 source "${ABSOLUTE_PATH}"/config.sh
 

setup.sh

 #!/bin/bash
 
-RELATIVE_PATH="$(dirname "${BASH_SOURCE[0]}")"
+SOURCE_PATH="${BASH_SOURCE[0]}"
+
+RELATIVE_PATH="$(dirname "$SOURCE_PATH")"
 ABSOLUTE_PATH="$(realpath "${RELATIVE_PATH}")"
 
 source "${ABSOLUTE_PATH}"/config.sh