conflicts with imported packages

```
newPackage("Intersection",
Version => "1.1",
Headline => "making Foo",
Configuration => { "foo" => 42, "bar" => "x" },
PackageImports => { "Polyhedra" },
DebuggingMode => true
)
export "intersection"
intersection = method()
```
The assignment above fails because Polyhedra defines "intersection", too. A warning message should be issued when the symbol is exported here.

Reproduce the issue with the shown newPackage("Intersection") example, importing Polyhedra and exporting intersection. Start by tracing how PackageImports and export are handled, then identify where imported symbols are compared with exported symbols. Done means the example emits a warning when the exported name conflicts with a name from an imported package.

Written by the indexing model from the issue text.