Merge multiple files into a single namespace

Migrated issue, originally created by Ben Beanfield (@benbeanfield)

It would be great if functions or tags defined in multiple template files could be added to the same namespace. This would allow for the creation of widget libraries without having to put every function or tag into one file in order to access them from the same namespace.

For example:

## /foo.mako
<%namespace name="forms" file="/forms/input.mako" inheritable="True">
<%namespace name="forms" file="/forms/textarea.mako" inheritable="True">
<%namespace name="forms" file="/forms/button.mako" inheritable="True">

## Each of the above template files defines a single def, which would be accessed with:

$(forms.input()}
$(forms.textarea()}
$(forms.button()}

Or alternatively:

## /foo.mako
<%namespace name="forms" files=${['/forms/input.mako', '/forms/textarea.mako', '/forms/button.mako']}>

$(forms.textarea()}

Further, it would be great if a single file could be used to import all of the individual template files into a single namespace, and then this single file could be used by other templates.

For example:

## /forms/index.mako
<%namespace name="forms" file="/forms/input.mako" inheritable="True" import="*">
<%namespace name="forms" file="/forms/textarea.mako" inheritable="True" import="*">
<%namespace name="forms" file="/forms/button.mako" inheritable="True" import="*">

## /foo.mako
<%namespace name="forms" file="/forms/index.mako">
$(forms.input()}
$(forms.textarea()}
$(forms.button()}

Start by reproducing the namespace examples in /foo.mako and /forms/index.mako with the individual /forms/input.mako, /forms/textarea.mako, and /forms/button.mako files. The work is done when multiple template files can expose their definitions through one namespace, including the index-file import example, without requiring all definitions in one file.