Fix getCurrentDirectory calls in ghcide

[pepeiborra]

lsp HEAD no longer sets the current directory to the workspace root (haskell/lsp#326) so we need to remove this assumption from the ghcide codebase. There are 3 places where this is relevant:

1. In the call to loadCradle, or rather, in the GHC session returned by it, where any relative paths in the flags returned by the cradle will expect to be resolved from the directory where the hie.yaml file lives - should change the working directory to the hie.yaml location
2. In the logic to setInitialDynFlags, where we search for a hie.yaml file in the current directory - should use the workspace root instead
3. In the LSP initialisation handler, where the current directory is included in the hiedb cache dir hash - should use the workspace root instead

[pepeiborra]

#1894 is open to test these changes with lsp HEAD

[berberman]

LGTM