cider-expected-ns returns nil on boot projects

[Malabarba]

This happens because the list of strings returned by (cider-sync-request:classpath) apparently doesn't include the project directory.
This, in turn, happens because boot masks the project under: `~/.boot/cache/tmp/FULL/PATH/ followed by some gibberish.

As a (hopefully) temporary patch, I'll make this function fallback on clojure-expected-ns.

[bbatsov]

This bites us every time. I'm guessing we can reuse some of boot-related code for the info middleware.

[grzm]

Digging into this a bit, it looks like the issue is related to the values returned by cider-sync-request:classpath. Boot added a :fake-class-path key to the boot environment (also available via the "fake.class.path" system property) to address issues like the one here. Some of this was addressed in clojure-emacs/cider-nrepl@fbcecba to access resource paths. Does it make sense for cider-sync-request:classpath to return :fake-class-path when running under boot? From what I can see, the only usage of this is in cider-expected-ns.

[grzm]

I've put together a POC here: grzm/cider-nrepl@4fe4a70. If this looks good, I'll submit a PR to cider-nrepl. (Should we continue this discussion under a ticket for cider-nrepl?)