system.file() shim function could take into account .Rbuildignore

[renozao]

When a package is loaded with devtools, system.file() still returns a valid path when called on a file that exists in the package source (typically under inst/) but would be absent on the installed package because of the application of the the filters in .Rbuildignore.

In order to better mimic what happens in the installed package, The shim function that devtools already defines to handle the extra inst/ sub-directory in source mode could apply .Rbuildignore to the result before returning and issue a warning about the fact that the file is present but excluded from the build, before failing or returning "" as per the normal behaviour.
This would help detecting this situation and point the developer to the issue, avoiding puzzling discrepancy between local source and CI/installation behaviors.

Thanks!!