Add support for conditions

[robwoolley]

Packages like osqp_vendor contain multiple build_type elements. These use conditionals to apply to both ROS 1 and ROS 2: https://github.com/tier4/osqp_vendor/blob/main/package.xml

This causes an exception in catkin_pkg.package:

ERROR:colcon:mash None: Only one <build_type> element is permitted.
Traceback (most recent call last):
File "/home/spaceros-user/mash/myenv/lib/python3.12/site-packages/colcon_core/command.py", line 593, in verb_main
rc = context.args.main(context=context)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/spaceros-user/mash/myenv/lib/python3.12/site-packages/mash/verb/bitbake.py", line 121, in main
pkg_metadata = PackageMetadata(package_manifest, None)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/spaceros-user/mash/myenv/lib/python3.12/site-packages/mash/PackageMetadata.py", line 75, in init
self.build_type = pkg.get_build_type()
^^^^^^^^^^^^^^^^^^^^
File "/home/spaceros-user/mash/myenv/lib/python3.12/site-packages/catkin_pkg/package.py", line 167, in get_build_type
raise InvalidPackage('Only one <build_type> element is permitted.', self.filename)
catkin_pkg.package.InvalidPackage: Only one <build_type> element is permitted.

Octomap and behaviortree_cpp also use conditions.