This commit deprecates profile management from the activation script.
The profile management is instead the responsibility of the driving
software, for example, the `home-manager` tool in the case of
standalone installs.
The legacy behavior is still available for backwards compatibility but
may be removed in the future.
The new behavior resolves (or moves us closer to resolving) a number
of long standing open issues:
- `home-manager switch --rollback`, which performs a rollback to the
previous Home Manager generation before activating. While it was
previously possible to accomplish this by activating an old
generation, it did always create a new profile generation.
This option has been implemented as part of this commit.
- `home-manager switch --test`, which activates the configuration but
does not create a new profile generation.
This option has _not_ been implemented here since it relies on the
current configuration being activated on login, which we do not
currently do.
- When using the "Home Manager as a NixOS module" installation method
we previously created an odd `home-manager` per-user "shadow
profile" for the user. This is no longer necessary.
This has been implemented as part of this commit.
Fixes#3450
