Improve SemIR naming of import_refs

[zygoloid]

Currently import_refs have a non-descriptive name, containing only a content hash and disambiguator, such as %import_ref.a69 or %import_ref.f9d349.2. We should be able to do better.

Ideas:

• Include the name of the package and library from which the entity was ultimately imported in the name. We may want to do some shortening here, for example we could take the component of the library name following the final /.
• When the import_ref is loaded, look at its constant value and use that to form a name for the import_ref.
• Use the instruction in the target IR to form a name.
• Cache InstNamers for all IRs being processed by the toolchain invocation, and actually use the name of the instruction (including disambiguator) from the target IR, either in the name of the import_ref or at least in the right-hand side when printing formatted SemIR.