improved verbose level message for debugging in ImportTask

While I worked on a phing solution to build-docs/package my pear packages, based on a series of modules all named build.xml into subdirectories, 

I found difficult to debug where come the problem while reading the log contents, because we always
have the basename of the script.

With the phing tests import scripts it's not difficult to find origin because we have 

    [import] Importing configuration from imported.xml
    [import] Importing configuration from importedImport.xml

But I'd prefere to have such results : 

    [import] Importing configuration from \home\github\phing\test\etc\tasks\imports\imported.xml
    [import] Importing configuration from \home\github\phing\test\etc\tasks\imports\importedImport.xml

Especially in case of base scriptname equals

- Laurent