Build package instead of single file

Creating source files under src/ is not allowed with go build,
so in new projects a "main"-package is created now,
and the new go-file is placed in there.

When "Build before Run is enabled", the package which will be built,
is determined from the Script-File chosen in the RunConfiguration.
This package will be built by "go build".

If all source-files are placed in the project-root, compilation still works.

#614 #555