Add creating workspace member MSI packages

[volks73]

The default create subcommand, i.e. cargo wix, should be updated to automatically create the MSI packages of workspace members when the cargo wix command is executed from root of the workspace. The create subcommand should inspect each workspace member for a wix\main.wxs file and and create the MSI if found, then if a wix\main.wxs file is found in the workspace root, it creates the bundle of all the MSI packages, or creates the global MSI package of the binaries.

This would add better support for bundles and workspaces.

[volks73]

See also #112 for more information.

[volks73]

I wonder about the interplay between the -p,--package option and the <INPUT> positional argument. What if both are used for the same package manifest? Does the existence of the <INPUT> positional argument, which takes a path to a Cargo.toml file, mean the -p,--package option is not needed? If an installer for a specific package within a workspace is to be created, then the path to the package's manifest can be passed via the <INPUT>. The -p,--package option seems redundant, but it could also just be a short cut. Does usage of the -p,--package option conflict with the <INPUT> argument?

Hmm, seems I am circling back to the discussion in #94.