More flexible search paths

[LecrisUT]

I would like to suggest some more flexible search paths and definitions:

• besides the /.tito location, it would be nice to search for /.distro/tito and its variants. Reasoning is that sometimes upstream does not want to populate the root with tool files and push files like README.md too far down
• expose a spec_file property pointing to the main spec file location
• similar property for SOURCEX search path
• add an optional positional argument to tito build [package_name] which is useful for tito build --test
• find package names from .tito/packages/<package_name> which can be passed in the above. Also use the spec_file property suggested above that can be overwritten for each .tito/packages/<package_name>/tito.props or equivalents to point to where the spec file is