refactor: refactor variable names in valid-{tags,repos}.sh`

Currently the variables exported by src/lib/valid-tags.sh are named _tags and _arch which leads to weird things like this (see !169 (merged)):

for loop_arch in "${arch[@]}"; do 
	if in_array "${loop_arch}" "${_arch[@]}"; then
		BUILD_ARCH+=("$loop_arch")
	else
		warning 'invalid architecture, not building for: %s' "${loop_arch}"
	fi
done

So the goal of this refactoring would be to rename _arch to something like DEVTOOLS_VALID_ARCHITECTURES or whatever and replace every usage in the codebase with the new chosen name.