rename package to spago-legacy

This package is currently `spago` in NixPkgs:
https://search.nixos.org/packages?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=spago

The Nix configuration for that is generated from this Cabal file.
Renaming to `spago-legacy` will help NixPkgs update to the new Spago.

I think it's unnecessary for the `spago` binary to be renamed.

spago.cabal → spago-legacy.cabal

@@ -1,10 +1,10 @@
 cabal-version: 2.4
 
-name:           spago
+name:           spago-legacy
 version:        0.21.0
-description:    Please see the README on GitHub at <https://github.com/purescript/spago#readme>
-homepage:       https://github.com/purescript/spago#readme
-bug-reports:    https://github.com/purescript/spago/issues
+description:    Please see the README on GitHub at <https://github.com/purescript/spago-legacy#readme>
+homepage:       https://github.com/purescript/spago-legacy#readme
+bug-reports:    https://github.com/purescript/spago-legacy/issues
 author:         Justin Woo, Fabrizio Ferrai
 maintainer:     @jusrin00, @fabferrai
 copyright:      2018-2020 Justin Woo, Fabrizio Ferrai
@@ -28,7 +28,7 @@ extra-source-files:
 
 source-repository head
   type: git
-  location: https://github.com/purescript/spago
+  location: https://github.com/purescript/spago-legacy
 
 common defaults
   default-language: Haskell2010
@@ -114,9 +114,9 @@ library
     Spago.Version
     Spago.Watch
   other-modules:
-    Paths_spago
+    Paths_spago_legacy
   autogen-modules:
-    Paths_spago
+    Paths_spago_legacy
   ghc-options: -Wall -Wcompat -Wincomplete-record-updates -Wredundant-constraints
   build-depends:
       Glob
@@ -175,14 +175,14 @@ executable spago
   import: defaults
   main-is: Spago.hs
   other-modules:
-    Paths_spago
+    Paths_spago_legacy
   autogen-modules:
-    Paths_spago
+    Paths_spago_legacy
   hs-source-dirs: app
   build-depends:
       ansi-terminal
     , base >=4.7 && <5
-    , spago
+    , spago-legacy
     , text <1.3
     , turtle
     , with-utf8
@@ -200,7 +200,7 @@ test-suite spec
     Spec
     UnitSpec
     Utils
-    Paths_spago
+    Paths_spago_legacy
   ghc-options: -Wall -Wcompat -Wincomplete-record-updates -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N -main-is Main
   build-depends:
       QuickCheck
@@ -212,12 +212,12 @@ test-suite spec
     , hspec-megaparsec
     , megaparsec
     , process
-    , spago
+    , spago-legacy
     , temporary
     , text <1.3
     , turtle
     , versions == 6.*
   build-tool-depends:
       hspec-discover:hspec-discover == 2.*
     -- we need the the executable available for the end to end tests
-    , spago:spago -any
+    , spago-legacy:spago -any