Support for relative paths in the source file launcher's paths; automatic expanding of modular paths.

[lahodaj]

The VS Code extension supports the (multi-)source file launcher using the RUN CONFIGURATION explorer tab. One can specify the command line options, which are then used when starting a file in the workspace.

There are a few problems with that:

• options like -classpath could contain relative paths, but NB will not resolve them
• the default working directory is weird - it is the directory in which the file-to-run resides. That made sense for single-source launcher, but not anymore. And while runtime will resolve relative paths, it will resolve them to this weird working directory
• for --module-path, the elements specified may be modules, or a directory, which contains the modules. But NB will only see module path which contains modules, and will not resolve modules inside the specified directory

This patch:

• when needed, computes "relevant" default working directory for files - it is a) the workspace folder that encloses the given file (which should be the common case), or, if that fails to find a workspace folder b) the workspace folder that is enclosed by the file's source root (for the case where the user opens just a part of the source root)
• this relevant default working directory is then used as the working directory, unless overridden by the user
• the working directory (either computed as above, or explicit) is used the base to resolve relative paths
• when a module path contains a directory which does not contain module-info.class, it is interpreted as a directory consisting of a collection of modules, and will expand the module path to include these modules. This includes listening on the directory content, and updates to the module path when the content changes