Add environment variable support for modules (#331)

* Add 'expor env' dummy command

* (WIP) Abstract away module exportables as Overlay

* Switch to Overlays for use/hide

Works for decls only right now.

* Fix passing import patterns of hide to eval

* Simplify use/hide of decls

* Add ImportPattern as Expr; Add use env eval

Still no parsing of "export env" so I can't test it yet.

* Refactor export parsing; Add InternalError

* Add env var export and activation; Misc changes

Now it is possible to `use` env var that was exported from a module.

This commit also adds some new errors and other small changes.

* Add env var hiding

* Fix eval not recognizing hidden decls

Without this change, calling `hide foo`, the evaluator does not know
whether a custom command named "foo" was hidden during parsing,
therefore, it is not possible to reliably throw an error about the "foo"
name not found.

* Add use/hide/export env var tests; Cleanup; Notes

* Ignore hide env related tests for now

* Fix main branch merge mess

* Fixed multi-word export def

* Fix hiding tests on Windows

* Remove env var hiding for now

crates/nu-parser/src/flatten.rs

@@ -1,4 +1,6 @@
-use nu_protocol::ast::{Block, Expr, Expression, PathMember, Pipeline, Statement};
+use nu_protocol::ast::{
+    Block, Expr, Expression, ImportPatternMember, PathMember, Pipeline, Statement,
+};
 use nu_protocol::{engine::StateWorkingSet, Span};
 
 #[derive(Debug, Eq, PartialEq, Ord, PartialOrd)]
@@ -133,6 +135,25 @@ pub fn flatten_expression(
             }
             output
         }
+        Expr::ImportPattern(import_pattern) => {
+            let mut output = vec![(import_pattern.head.span, FlatShape::String)];
+
+            for member in &import_pattern.members {
+                match member {
+                    ImportPatternMember::Glob { span } => output.push((*span, FlatShape::String)),
+                    ImportPatternMember::Name { span, .. } => {
+                        output.push((*span, FlatShape::String))
+                    }
+                    ImportPatternMember::List { names } => {
+                        for (_, span) in names {
+                            output.push((*span, FlatShape::String));
+                        }
+                    }
+                }
+            }
+
+            output
+        }
         Expr::Range(from, next, to, op) => {
             let mut output = vec![];
             if let Some(f) = from {