home-manager: Add --flake option to home-manager (#1856)

Implements a --flake options for build and switch, along with the usual
flake related optons (for lock-files etc).

Configurations in the flake are automatically discovered in the
following order:
1. `outputs.homeConfigurations."$flake-uri"` (the `--flake parameter`)
2. `outputs.homeConfigurations."$USERNAME@$HOSTNAME"`
3. `outputs.homeConfigurations."$USERNAME"`

Make home-manager use default configuration from
~/.config/nixpkgs/flake.nix, if it exists and nothing else is
specified.

Co-authored-by: Nicolas Berbiche <nicolas@normie.dev>

doc/man-home-manager.xml

@@ -66,6 +66,10 @@
     -I <replaceable>path</replaceable>
    </arg>
     
+   <arg>
+    --flake <replaceable>flake-uri</replaceable>
+   </arg>
+    
    <arg>
     -b <replaceable>ext</replaceable>
    </arg>
@@ -160,7 +164,7 @@
   <title>Description</title>
   <para>
    This command updates the user environment so that it corresponds to the
-   configuration specified in <filename>~/.config/nixpkgs/home.nix</filename>.
+      configuration specified in <filename>~/.config/nixpkgs/home.nix</filename> or <filename>~/.config/nixpkgs/flake.nix</filename>.
   </para>
   <para>
    All operations using this tool expects a sub-command that indicates the
@@ -343,6 +347,18 @@
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+    <term>
+     <option>--flake <replaceable>flake-uri[#name]</replaceable></option>
+    </term>
+    <listitem>
+     <para>
+      Build Home Manager configuration from the flake, which must contain the
+      output homeConfigurations.name. If no name is specified it will first try
+      username@hostname and then username.
+     </para>
+    </listitem>
+   </varlistentry>
    <varlistentry>
     <term>
      <option>-b <replaceable>extension</replaceable></option>