Get logical imports in "wren_modules" working.

There's a lot of changes here and surely some rough edges to iron out.
Also, I need to update the docs. But I want to get closer to landing
this so I can build on it.

test/language/module/compile_error/compile_error.wren

@@ -1,2 +1,2 @@
 System.print("before") // expect: before
-import "./module" for Module // expect runtime error: Could not compile module 'test/language/module/compile_error/module'.
+import "./module" for Module // expect runtime error: Could not compile module './test/language/module/compile_error/module'.

test/language/module/logical_dir/logical_dir.wren

@@ -0,0 +1,6 @@
+// Import a module from within a named package.
+import "foo/within_foo" for Foo
+// expect: ran foo module
+// expect: ran bar module
+
+System.print(Foo) // expect: from foo

test/language/module/logical_dir/wren_modules/bar/within_bar.wren

@@ -0,0 +1,5 @@
+// nontest
+var Bar = "from bar"
+System.print("ran bar module")
+
+import "foo/within_foo"

test/language/module/logical_dir/wren_modules/foo/within_foo.wren

@@ -0,0 +1,5 @@
+// nontest
+var Foo = "from foo"
+System.print("ran foo module")
+
+import "bar/within_bar"

test/language/module/logical_short/logical_short.wren

@@ -0,0 +1,5 @@
+// Import a module whose name is the same as the package.
+import "foo" for Module
+// expect: ran module
+
+System.print(Module) // expect: from module

test/language/module/logical_short/wren_modules/foo/foo.wren

@@ -0,0 +1,3 @@
+// nontest
+var Module = "from module"
+System.print("ran module")

test/language/module/unknown_module.wren

@@ -1 +1 @@
-import "./does_not_exist" for DoesNotExist // expect runtime error: Could not load module 'test/language/module/does_not_exist'.
+import "./does_not_exist" for DoesNotExist // expect runtime error: Could not load module './test/language/module/does_not_exist'.

test/language/module/unknown_variable/unknown_variable.wren

@@ -1,3 +1,3 @@
 // Should execute the module:
 // expect: ran module
-import "./module" for DoesNotExist // expect runtime error: Could not find a variable named 'DoesNotExist' in module 'test/language/module/unknown_variable/module'.
+import "./module" for DoesNotExist // expect runtime error: Could not find a variable named 'DoesNotExist' in module './test/language/module/unknown_variable/module'.

test/language/module/use_nearest_modules_dir/a/b/use_nearest_modules_dir.wren

@@ -0,0 +1,3 @@
+// Stops as soon as it finds a wren_modules directory, regardless of whether or
+// not it contains the desired module.
+import "foo" // expect runtime error: Could not load module 'foo'.

test/language/module/use_nearest_modules_dir/wren_modules/foo/foo.wren

@@ -0,0 +1,2 @@
+// nontest
+System.print("ran foo module")

test/language/module/walk_up_for_modules_dir/a/b/walk_up_for_modules_dir.wren

@@ -0,0 +1,3 @@
+// Walk up parent directories from the root script to find "wren_modules".
+import "foo"
+// expect: ran foo module

test/language/module/walk_up_for_modules_dir/wren_modules/foo/foo.wren

@@ -0,0 +1,2 @@
+// nontest
+System.print("ran foo module")