specl: isolate specl modules into their own namespace.

To avoid potential clashes with modules along the load-path of a
specl client, refer to specl's own modules using the specl name-
space.
* specl/specl.in: Use `require "specl.std"`.  specl.lua is
already in the specl namespace, so no need to prefix that one.
* specl/specl.lua: Likewise for specl modules required here.
* specl/std.lua: Likewise.
* specl/specl.mk (SPECL_PATH): Support prefixed requires in the
build tree.
(specl_install_edit): Likewise, for the install tree.
(nobase_dist_lua_DATA): Move specl/specl.lua from here...
(dist_lua_DATA): ...to here, so that specl.lua is not installed
into the specl subdirectory, and `require "specl"` works
correctly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>