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Have a built-in mechanism to display all available import libraries within the interpreter. Like what is available through the built-in dir function.

>>> import sys
>>> sys.modules.keys()
['google', 'copy_reg', 'sre_compile', '_sre', 'encodings', 'site', '__builtin__', 'sysconfig', '__main__', 'encodings.encodings', 'abc', 'posixpath', '_weakrefset', 'errno', 'encodings.codecs', 'sre_constants', 're', '_abcoll', 'types', '_codecs', 'encodings.__builtin__', '_warnings', 'genericpath', 'stat', 'zipimport', '_sysconfigdata', 'warnings', 'UserDict', 'encodings.utf_8', 'sys', 'codecs', 'readline', 'os.path', 'signal', 'traceback', 'linecache', 'posix', 'encodings.aliases', 'exceptions', 'sre_parse', 'os', '_weakref']

This is not what the OP asked, AFAICT. sys.modules is just the set of currently imported modules, not all potentially importable modules.

I guess I have no idea what the OP is asking for then. dir() certainly only gives immediately available builtins.

+1 for the OP's original idea (something like a dir_modules() function that lists out all possible imports).
I think something like this must already be present in some of the Python front-ends such as ipython and bpython. That is how they support tab-completion for imports:
In [1]: import co
code codeop colorsys compileall contextlib copy
codecs collections commands compiler cookielib copy_reg
In [1]: import re
re readline repr requests resource reverend rexec

The trick would be how to query finders to say "what could you find?" There is no API for that so either something pragmatic that won't work in the face of e.g. zipfiles would need to be used or a new optional API on finders to list what the tail name of modules it can find are (but without the package name as most finders just use fullname.rpartition('.')[-1] to figure out what they are looking for).

I wrote some code that does something like this for a hacky custom readline completer. See the import_list() method at <https://github.com/vadmium/etc/blob/6ac333f/python/pythonstartup.py#L222>. It looks like I’m using a combination of "sys.builtin_module_names" and pkgutil.iter_modules(), with a whole lot of compatibility and bug workarounds.

When a replacement for pkgutil.walk_packages() is added to importlib to work appropriately with the import system then this should be doable. See https://bugs.python.org/issue19939 for work to replace pkgutil as appropriate.