gvle: fix the plugin file generation

gvle assume that the folder loaded is a project working directory. It
does keep it as a member field. And when a plugin does want to save a
generated file, the expected place is the "src" subdirectory of the
project source directory (Closes #63,#62).
gvle: add before to simulate always build

The opportunity to always configure, build and install the package has
been made available. This feature can be disable with the new
configuration switch gvle.packages.auto-build.