Apply resolve hook to entry module

[Rich-Harris]

not ready for merge

This enables #30, and by extension #25 (eventually), by applying the resolvePath hook to options.entry. resolvePath is now a misnomer; it should become resolveId or something similar, and there are a few other bits of internal terminology that will probably need updating as well.

Something to note: bundle.entryPath and bundle.base no longer exist. These were used for the purposes of generating non-conflicting identifiers by restricting the 'available' path parts to those within the source directory (i.e. _foo to deconflict with foo, rather than Users_rharris_code_someProject_src_foo) - for now, that's being worked around, but it'll need a more robust approach eventually.

• fix internal terminology
• alternative approach to bundle.base
• update docs

[guybedford]

I guess bundle.base is something used by the default id resolver? Surely it can still exist in that way though?

Perhaps the resolveId name change can be a later breaking change, and a first step can get the functionality in minimally breaking?

[Rich-Harris]

Wow, was it really three weeks ago that I opened this PR? 😵 Anyway, this is released as 0.9.0 - resolveId has taken the place of resolvePath (I'd already made this change locally - figured it's better to try and do it right in one go), and is applied to all modules including the entry module, so this should all work now. I'm glossing over the bundle.base stuff for the time being.

[guybedford]

Awesome! Will test it out very soon. It should even allow self-hosting SystemJS's build in future. 🐶 🍕