A couple of improvements on the handling of project files and paths (#463)

A couple of improvements on the handling of project files and paths (#463)

* Alire.Releases: fix basename in Project_Files()

* Alire.Releases.Project_Paths: Add empty path for gpr in the root dir of a crate

* Alire.{Root|Release}: remove trailing path separator in Roots.Project_Paths

Doing it here means that case "base_dir/<empty_path>" is also handled.
i.e. return "base_dir" instead of "base_dir/"

* Alire.Roots: Do not always add root dir of crates in Project_Paths

The root dir will be included when necessary, i.e. when there is a project file.

* testsuite/drivers/asserts.py: revert repr() on output compare

It was useful to detect \r\n vs \n but make the test failure very
difficult to analyze otherwise.

* GPR_PROJECT_PATH tests: fix Windows vs UNIX paths

* tests/auto-gpr-with/gpr_in_subdir: improve test explanation

* Alire.Roots: expect UNIX dir separators from crates