SI-8403 Fix regression in name binding with imports in templates

Regressed in dbd8457 which changed `Context#make` to automatically
include the imports from the given `Tree` if it was an `Import`
tree, rather than requiring callers to call `makeNewImport`.

However, this turns out to double up the imports for the "inner" namer
of a template that starts with imports. The inner namer has a new
scope, but the same owner and tree as its parent.

This commit detects this case by seeing if the `Import` tree used
to consruct the child context is the same as the parent context.
If that is the case, we don't augment `Context#imports`.

Review by @adriaanm