Refactoring: Adhere better to conventions.

Define function sdk in functions/sdk.fish and
perform init in conf.d/sdk.fish.

functions/sdk.fish

@@ -0,0 +1,27 @@
+set sdkman_init "$HOME/.sdkman/bin/sdkman-init.sh"
+
+# Guard: SDKMAN! needs to be installed
+if not test -f "$sdkman_init"
+  exit 0
+end
+
+# Declare the sdk command for fish
+function sdk -d "Manage SDKs"
+    # We need to leave stdin and stdout of sdk free for user interaction.
+    # So, pipe PATH (which might have changed) through a file.
+    # Now, getting the exit code of sdk itself is a hassle so pipe it as well.
+    # TODO: Can somebody get this to work without the overhead of a file?
+    set pipe (mktemp)
+    bash -c "source $sdkman_init && sdk $argv; echo -e \"\$PATH\n\$?\" > $pipe"
+    set bashDump (cat $pipe; rm $pipe)
+
+    set bashPath $bashDump[1]
+    set sdkStatus $bashDump[2]
+    # If SDKMAN! succeeded, copy PATH here (might have changed)
+    if [ $sdkStatus = 0 ]
+        set newPath (string split : "$bashPath")
+        set -gx PATH $newPath
+    end
+
+    return $sdkStatus
+end