Check if file is present instead of directory

Fixes #8886.

Problem: any `sync-all` command that is run before the first succesfull
`sync-all get` would trigger a false warning about an old `time` package
being present.

Cause: after cloning the ghc repository, the (empty) `libraries/time`
directory is already present.

Solution: check if the directory actually contains any of the `time`
files. I picked the `LICENSE` file, since `boot` does so as well.

Signed-off-by: Austin Seipp <austin@well-typed.com>