Improve the speed of file-only imports

[Gabriel439]

This overhauls exprFromPath (and every function downstream of that) to
speed up imports when no URLs are imported.

Previously, Dhall would always demand a Manager on the very first
import, whether or not Dhall actually needed to fetch a remote URL.
Fetching this Manager adds an unusually long overhead (~1 s) to
programs that only needed to fetch local files.

Now the Manager is only requested as late as possible (i.e. right
before the first URL import)

This is a breaking change, since it changes the types of several exported
utilities in the API