Specify that nested paths shall be absolute

[ghost]

In the section on run.files, add the requirement that when a URI-valued property refers to a nested file whose location can be specified by a path relative to its parent (for example, a file within a compressed archive), the nested path should be specified as an absolute path (that is, it should begin with a slash).

Right:

"uri": "file:///C:/packages/foo.zip#/bar/baz/qux.cpp"

Wrong:

"uri": "file:///C:/packages/foo.zip#bar/baz/qux.cpp"

It doesn't actually matter, but the idea is that this is a location starting from the "root" of the parent file.