Case insensitive LSB name checking

[sloretz]

This PR fixes a bug that breaks rosdep when using Python 3.8 on Ubuntu.

platform.linux_distribution(full_distribution_name=0) returns Ubuntu while distro.linux_distribution(full_distrition_name=0) returns ubuntu. This causes rosdep to fail to detect the os version on ubuntu when using python 3.8. This does not appear to be a bug in distro, which warns the data returned may differ.

Python 3.8.0 (default, Oct 28 2019, 16:14:01) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import distro
>>> distro.linux_distribution()
('Ubuntu', '18.04', 'bionic')
>>> distro.linux_distribution(full_distribution_name=0)
('ubuntu', '18.04', 'bionic')
>>> 

Python 3.7.5 (default, Nov  7 2019, 10:50:52) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.linux_distribution()
__main__:1: DeprecationWarning: dist() and linux_distribution() functions are deprecated in Python 3.5
('Ubuntu', '18.04', 'bionic')
>>> platform.linux_distribution(full_distribution_name=0)
('Ubuntu', '18.04', 'bionic')