Replace typematch.Itab?

[quasilyte]

I'm not sure we need this enter/leave scope semantics.
Now imports are func-scoped and there is no way to create a fine-grained scoping control of imports.
Docs may need to be updated accordingly.

A simple map[string]string should be good enough.
The same map can be then passed to gogrep as gogrep.CompileConfig.Imports.