Correct imports with dots and fix documentation of import behaviour

[Mr-Pine]

While the documentation in the README claims that an import of A.B.C is resolved to A/B/C.gr while the current code actually resolves it to A.B.C.gr. This PR changes the code behaviour to be on par with the documentation. It adittionally updates the README to explain the two-stage import resolution, first in the cwd, then in the include path.

[Mr-Pine]

As far as I can tell these build/test failures are unrealted to my change, but please correct me if I'm wrong

[dorchard]

Thanks for this. We have a hackathon coming up soon so we can review this PR then. Indeed, there are a bunch of tests failing but that's our fault!

[BinderDavid]

I verified locally on my PC that this now works.

On main:

> gr examples/foo.gr 
Checking examples/foo.gr...
Parse error: examples/foo.gr:
StdLib/Data.Bool.gr: openFile: does not exist (No such file or directory)

On this branch:

> gr examples/foo.gr 
Checking examples/foo.gr...
OK, evaluating...
False

with a file foo.gr

import Data.Bool

f : Bool
f = not True

main : Bool
main = f