Load new per-release index manifests (#479)

Load new per-release index manifests (#479)

* Migrate testsuite indexes to new split manifests

* Alire.Index: update community version

* Refactor Alr.OS_Lib.Traverse_Folder into Alire

As Alire.Directories.Traverse_Tree

* Alire.TOML_Index: traverse new folder structure

Also adapt the checks to the new structure, and double-check that the file name
matches the manifest contents (name, version).

* Refactor Crate internals for new flat manifests

Since now each release is truly independent, crates need not to store the
common information as previously. This is done now only for external
definitions in a narrower scope.

Several loading functions can also be streamlined since now it's simpler to get
a proper complete release without requiring its enclosing crate.

The bulk of the refactoring emanates from merging Alire.Crates.With_Releases
into simply Alire.Crates, and repurposing the Alire.Crates.Sections enum.

* Use new [[depends-on]] dependencies

Nothing outwards has changed yet; the manifest is still entirely machine
generated. This commit only changes the table [depends-on] into an array of
[[depends-on]] tables.

* Testsuite fixes for new format

Fixes in code detected by the testsuite, and small changes in output due to
code refactorings.

* New tests for index structure

* Review cleanup

Assorted minor refactors

* Code review: restore lost comments in testsuite