qdoc: Use lowercase names for modules read from index files

The module names are used as keys in the map that defines the
forest of module trees. A physical module name (lowercase) is
used as the primary tree name, but modules read from index
were using a name written to the index file (project name,
typically CamelCase) as-is.

This caused issues for resolving link commands that have a
module name in the square bracket parameters. To ensure that
such links work, convert module names read from index to
lowercase, too.

Change-Id: I698e01fd1df888e4a78c06a76ad72b5eb1a70892
Reviewed-by: Martin Smith <martin.smith@digia.com>