Allow overriding when an optional import took place

Hi!

I would like to be able to detect within a justfile whether an optional import took place.

I found out a workaround to do that within recipe bodies, namely
```just
# ./maybe.just
set export

IMPORTED := "true"

# ./justfile
import? 'maybe.just'

imported:
${IMPORTED:-false} && echo yes || echo no
```
however I did not find a way to have that information available in expressions.

If there's no way currently, here's a couple of things I tried that might be good features in their own right, and would solve this problem:
* when `allow-duplicate-variables` is set, one could have the position of the import be relevant for overriding. Reading the documentation of `allow-duplicate-variables`, I actually expected this to work:
```just
# ./maybe.just
imported := "true"
# ./justfile
imported := ""
import? "maybe.just"
default:
echo {{ if imported == "true" { "yes" } else { "no" } }}
```
If imports are allowed anywhere in a just file, I would expect duplicates from imports to override previous definitions (if duplicates are allowed).
If this held for recipes as well, I would probably even not need to use an `if`, and would just overwrite some recipes in `maybe.just`.
```just
# ./maybe.just
default:
echo yes
# ./justfile
default:
echo no
import? "maybe.just"
```
Afterwards I could see this goes directly in contrast with the way imports are specified in the documentation... so maybe this route could be taken with a new directive like `include`/`include?`?

* a `?` postfix operator could allow using undefined variables: `var?` would evaluate to the value of `var` if `var` is defined, and to `""` otherwise.
```just
# ./maybe.just
imported := "true"
# ./justfile
import? "maybe.just"
default:
echo {{ if imported? != "" { "yes" } else { "no" } }}
```
* an `imported` function. It could be specified to return `"true"` for modules that were imported
```just
# ./maybe.just

# ./justfile
import? "maybe.just"
default:
echo {{ if imported('maybe.just') == "true" { "yes" } else { "no" } }}
```

* for completeness, though I don't particularly like such a solution, an optional variable name after an `import?` could be filled with `"true"` or `""` depending on whether the import took place
```just
# ./maybe.just

# ./justfile
import? "maybe.just" imported
default:
echo {{ if imported == "true" { "yes" } else { "no" } }}
```

Start by examining how optional `import?` directives, duplicate variables, and expression evaluation are currently handled. Compare the proposed undefined-variable, imported-function, and import-order approaches, then determine which behavior should be specified and tested before implementation.

Written by the indexing model from the issue text.