new-repl creates non-satisfied .ghc.environment specification (cabal 2.1)

[lspitzner]

reproduce:

1. create empty lib-only package "testpackage"
2. cabal new-configure -w $MYGHCPATH
3. cabal new-repl

env file looks like this; note that the "inplace" entry is not satisfied until we new-build:

> cat .ghc.environment.x86_64-linux-8.0.2 
-- This is a GHC environment file written by cabal. This means you can
-- run ghc or ghci and get the environment of the project as a whole.
-- But you still need to use cabal repl $target to get the environment
-- of specific components (libs, exes, tests etc) because each one can
-- have its own source dirs, cpp flags etc.
-- 
clear-package-db
global-package-db
package-db $HOME/.cabal/store/ghc-8.0.2/package.db
package-db dist-newstyle/packagedb/ghc-8.0.2
package-id testpackage-0.1.0.0-inplace
package-id base-4.9.1.0
package-id ghc-prim-0.5.0.0
package-id rts
package-id integer-gmp-1.0.0.1