Fixes/tests for recursive link pins (#751)

Fixes/tests for recursive link pins (#751)

* Fixes for recursive link pins

* Use link relative paths in output

Internally absolute paths are used, which allows to remove some complex logic.

* Bypass dangling reference warning in some gnat versions

* Fixes for absolute paths on Windows

* Cosmetic fix for `alr pin`

* Ensure user pins in memory use absolute paths

This simplifies internals when referring to recursive pins. Paths in the
lockfile and displayed to the user are still shown as relative to the root.

* New improved remote pin management

* Remove some redundant info output

* Remove Externals.Softlinks in favor of User_Pins

* Found cause & workaround of CE2021 path corruption bug

* Testcase minor fix

* Fix spellcheck CI

I broke it inadvertently when disabling spelling for the lockfile

* New tests for recursive pins and more

* Clarifications and tweaks during self-review

* Backport VCSs.Git changes

* Update pins in alire.toml for self-build

* Fixes found by the testsuite on Windows

* Cosmetic fixes found during self-review