Part of #1137.

* Fix bugs causing spurious diffs on update

Solutions were the same, but some types internally where using pointers instead
of pointed objects for comparison.

WIP

* Don't attempt redundant system package installation

* Inform of pin checkouts/updates as for regular crates

* Ensure created folders remain the same

* Fix testsuite index for deterministic make version

That mv operation fails from time to time for unknown reasons.

* New workflow to check GNAT CE builds

* Fix compilation with CE 2020

* Document binary origins

* Test for explicit binary origin

* Report the system package manager in `alr version`

* Also report distro detection disabling

* Use a separate system deployer in unknown systems

Instead of defaulting to apt.

* Distinguish undetected exec from unknown exec name

* Explicitly check all GCC versions we want

* Remove unneeded `aliased`

* Try to use safer reference

* Workaround for GNAT 13 troubles with expr. funcs.

* Fix test index to use deterministic external gnat version

* Test for automated publishing

This test is dependent on a remote repo (`test-index`) on which a PR is opened
during testing, so we better run this one sporadically and locally, or in a
single selected CI configuration.

* Self-review

* Use `ALIRE_TESTSUITE_*` for testsuite env variables

* fix: tmp dir being defined but not existing

* Improved log message for temp file creation

* Prevent deleting a tmp folder not created by us

The shortcut from the Windows installer was setting a background color for PowerShell command prompt. The idea was to match the look of the PowerShell you would normally start, and avoid confusion with the CMD prompt which has a black background. But that didn't work very well as the background color just disappeared when running commands. Also it seems like Windows now uses a black background for the PowerShell anyway.

* Fix bugs causing spurious diffs on update

Solutions were the same, but some types internally where using pointers instead
of pointed objects for comparison.

WIP

* Separate style checking from experimental Ada switches

Bugfix: exception when trying to select a tool without the assistant while the configured one is missing (#1551)

* Fix exception when trying to select a toolchain

This problem arised only if trying to select a toolchain with --select <tool>
while the configured tool was missing on disk.

* Test for fix

* Fix improper regex in rpm-based detector

* Test for bugfix

* Document pending breaking changes

These require an `alr` or index version bump

* Self-review

* Test that fails prior to bugfix

* Fix for cache invalidation

An unrelated bug is fixed that happened when not using an actual binary crate
for a compiler in the testsuite.

Discard $XDG_CACHE_HOME in favor of $XDG_DATA_HOME, as some distro guidelines
erroneously propose to mount the cache on tmpfs.

* Report the name of an unreadable directory.

  * src/alire/alire-index_on_disk.adb (New_Handler.Process_Local_Index):
      If the Path isn't a readable directory (e.g. because it doesn't
      exist), add it to the message in Outcome_Failure.
  * testsuite/tests/index/local-index-not-found/test.py: updated for
      new 'not a readable directory' report format.

* Handle Windows separator doubling.

Document --chdir in user-changes

* Allow solving with unistalled tools as fallback

* Fix precedence of installed compiler over remote ones

* Add warning in solution diff about toolchain download

* Fallback on installed compiler over uninstalled one

This is after checking that the preferred compiler cannot fulfill the compiler
dependency.

* Tweaks to restore former behaviors

* Fix test that inadvertently used virtual crate

It should have been real all along without alternatives

* Rebase tweaks

* Reword information message

* Move configuration loading earlier

Also make sure it's impossible to use configuration before the proper path is
set.

* New test

* Better encapsulation

* Windows fix

* New warning for externally selected gnat

* New test