Unbreak test package-get-version

Hello Daniel, The test `package-get-version` can fail if the directory storing the elisp files is not called "compat". The steps to reproduce should be the following : ```shell git clone https://github.com/emacs-compat/compat randomdirectoryname cd randomdirectoryname make test ``` Only this : ```shell git clone https://github.com/emacs-compat/compat cd compat make test ``` works. This is because `package-get-version` relies as a last resort on the directory name to get the "good" main file of the package. I propose to set the name of the "main file" from which to read the version number ourselves. It is always the exact source file providing the compat feature required by the tests, so it should work every time. Then we can get the version number of the package like `package-get-version` would. Of course, there are probably multiple other ways to get the version number of the package, like some user-facing function. This is just one way of doing it. Best, Aymeric