plugin, containertool: Remove unnecessarily broad Foundation imports

[euanh]

Motivation

In many places we import a whole module but only use one or two structs or classes. In these cases, importing just the parts we need makes dependencies clearer.

Modifications

• Where only one definition is used, constrain the import statement to only that definition.
• Remove a couple of Foundation imports which were completely unused.

Result

No functional change. Dependencies are clearer.

Test Plan

Tests continue to pass.