lib: Fix dependencies on SPDX code

lib: Fix dependencies on SPDX code

The SPDX library code was being ignored from taskhash calculations due
to accidentally being omitted from BBIMPORTS. This meant that changes in
the code or dependent variables would not cause the task to rebuild
correctly.

In order to add spdx_common, convert the `Dep` object from a named tuple
to a frozen dataclass. These function more or less equivalently, but the
bitbake code parser cannot handle named tuples.

Finally, the vardepsexclude that used to be present on the recipe tasks
needs to be moved to the python code in order for the variables to be
correctly ignored. Several unused exclusions were removed

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>