Make .x. substitution safer

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-dots@2172 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
o filePath("./././././") now returns "." (was "").  Added package system tests for filePath().

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-dots@2173 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
o Now skipping missing expressions via tryCatch().

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r-dots@2174 edb9625f-4e0d-4859-8d74-9fd3b1da38cb