fixes rospack export for package with condition attrib

[k-okada]

For example

$ rospack export --lang=cpp --attrib=cflags eigenpy

returns error with

[rospack] Error: package 'eigenpy' depends on non-existent package 'ament_cmake' and rosdep claims that it is not a system dependency. Check the ROS_PACKAGE_PATH or try calling 'rosdep update

If we have #113, it will outputs the result, but on melodic, we can not run export if for example eigenpy and other packages wihch skip rosdep rule by condition attribute, failing

This pr check condition attribute and evaluate them

[jacobperron]

Unfortunately, I think it's a bit more complicated than assuming we have exactly the pattern "lhs op rhs". According to spec, spaces are optional and parentheses are allowed too. So, I don't think the following input will work out as you might expect: "$ROS_VERSION==1"

Also consider a more complex example (I think is valid):

"($ROS_VERSION == 1 and $FOO != bar) or $FOO == baz"