Include is_lazy in import details

[seddonym]

Prior to this, explicit lazy imports were no different from normal imports. This PR:

• Adds an is_lazy key to the DetailedImport dict returned by get_import_details.
• Adds an optional is_lazy argument to add_import.

Still to do

Caching

Figure out how this should work with caching.

Backward compatibility for add_import

Tweak the behaviour for add_import so it's backward-compatible.

• It should be possible to call add_import in the old style, both with and without the old optional arguments (line_number / line_contents). If called with the optional arguments, is_lazy should be False.
• It should be possible to call with is_lazy. It probably makes sense in that case also to require line_number / line_contents. Might be worth taking the opportunity to raise a better exception if this isn't followed.