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For testing, libdocument can use a "EV_BACKENDS_DIR" environment variable to select the directory to read the modules from, allowing simple and fast testing. However, the loading behavior relies on the ".evince-backend" files to be in the same location as the modules. Previous to this commit, the backend modules where built under "backend/{name}", whereas their .evince-backend files used as metadata were installed just in "backend". This made testing backend changes impossible without either modifying the meson-generated structure or temporarilly installing the locally-built evince. This commit fixes that by having the shared_modules built in the top "backend" directory Additionally, it reduces code duplication along meson files. Installing the ".evince-backend" files in their corresponding backend without duplication in every backend is not possible, since meson does not allow paths in the output of configuration_file instructions.
