Allow tracing of keywords if imported through a parameterized resource import.

[dkindel]

While I know the title is probably confusing, what I mean is that suppose we have an import like this:

Resource ../environment.robot Resource ${RESOURCES}/variables/metadata.robot

Keywords or variables from metadata.robot can be followed by hitting F4. Likewise, following those variable declarations like ${RESOURCES} where it's defined in environment.robot would be desired.

Currently, only keywords where we're directly given a path to the resource can be followed with F4.