flag filenames in RUN and CALL that do not resolve when loading from String literals

[StephanWald]

File names in CALL and RUN are passed as strings. When they are in the source code as String literals RUN "myprog.bbj" and myprog.bbj cannot be resolved in the PREFIX, flag the filename as warning (not error). Do not flag if the String is built dynamically from variables (note: BBUtils.bbj exists in the PREFIX in the BBj install.)

use ::BBUtils.bbj::BBUtils

REM this file does not exist:
use ::BBUtils_xx.bbj::BBUtils_xx

REM these lines would be okay, they will work at runtime
RUN "BBUtils.bbj"
CALL "BBUtils.bbj"

REM these file names do not exist in the project. Should be flagged as as warning (NOT hard error)
RUN "BBUtils_xx.bbj"
CALL "BBUtils_xx.bbj"

REM to be considered: these Strings that resolve to file paths can be dynamic. This should never be flagged:
A$="BBUtils.bbj"
RUN "./"+A$
CALL "./"+A$