This represents the first step in migrating `home.file` to support
arbitrary absolute paths. This is to allow Home Manager to manage
files anywhere, provided the user has sufficient privileges.
For example, with this change the configuration
home.file."test/one".text = "foo";
home.file."/test/two".text = "foo";
will result in the files "$HOME/test/one" and "/test/two",
respectively. Note, a relative file name will still be relative
`$HOME`.
To allow a reasonable transition between the old and new path handling
we introduce the notion of "generation directory layout version". The
version is simply a file `version` within the generation directory
containing a number indicating the version number.
The version 0 (also implied if the version file is missing) indicates
the legacy layout where managed file paths always are relative
`$HOME`.
Version 1 indicates the new layout where managed file paths are
relative `/`.
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