Rework module system

Don't require/allow naming a module, instead require that the importer
names it. Allow importing into the local scope with the name !Local.
Simplify namespacing logic. Updates all tests to reflect these changes.

test/named-imports/1.tri

@@ -0,0 +1,7 @@
+
+!import "lib/base.tri"
+
+!import "test/named-imports/2.tri"
+!import "test/named-imports/3.tri" ThreeRenamed
+
+main = equal? (equal? Two.x 2) (equal? ThreeRenamed.x 3)

test/named-imports/2.tri

@@ -0,0 +1,2 @@
+
+x = 2

test/named-imports/3.tri

@@ -0,0 +1,2 @@
+
+x = 3