[JuliaLowering] Clean up `import` desugaring (#62142)
Cleans some dead code after the EST change (we no longer need to handle
quoted stuff in the importpath). More importantly, use the `K"inert"`
representation instead of an Expr in `K"Value"`; I'd like to disallow
Expr-AST in `K"Value"` entirely (it's best not to allow two
representations of the same thing), but there are a couple remaining
generated-function-interpolation cases I haven't touched in this change.
Also expands test coverage (tests partially Clauded)
