Do not crash when source plugins conflict

Fixes #3015 and #3180.
Use case in buildmessage titles consistently

Job names should not be capitalized (so they look OK in "While xxx"
messages), and are capitalized by the progress bar.

Fixes #3003.
Don't crash on circular dependency

Previously, we would register the circular dependency error properly
with buildmessage, but then try to build the package with a circular
dependency anyway, leading to a crash.

Fixes #3280.
Don't compile web.cordova unibuilds unless needed

Specifically, we only compile them if there's a cordova platform in the
current project, or if we are publishing the package.

(Ideally, we wouldn't require every published package to have
web.browser and web.cordova unibuilds --- we'd just publish a 'web'
unibuild unless there's actually a difference between the two. But we
are not there yet.)

This adds an extra flag to isopack-buildinfo.json, so that we know to
rebuild all the isopacks when we add the first cordova platform (or
remove the last cordova platform).

The implementation around publish is a little clunky; if you're in a
non-Cordova app and run meteor publish, it will rebuild all the packages
with web.cordova, and the next time you prepare the app it will rebuild
them again without it. It does work though.

Fixes #3274.
publish in app: don't write, display versions

Typically, there is a delta due to adding the test slice of the
published package, but there's no reason to tell the user about this or
save it in .meteor/versions.
Add the promise NPM package to the dev bundle.
Fall back to fs.watchFile if pathwatcher.watch does not work.

Fixes #3284.
Allow forcing fs.watchFile with an environment variable.
Factor out almost all fs. and path. calls in the tool

This will be useful when we want to be smart with windows file paths later
Also, all of the file calls are asynchronous with fibers now, which comes with
many benefits.

This is a combination of 23 commits. Original messages:
Wrap a large number of fs calls inside files.*

Convert a few more fs calls to files.*

More moving fs.* to files

Implement read/write streams and open/read/close

Get rid of fs from auth.js

Remove fs and unused imports from catalog-local and catalog-remote

Remove unused imports from catalog.js

Replace a whole lot of fs calls

Fix error

Migrate a lot more fs. calls to files.

Add a temporary symlink method

Convert old test to files.*

Use files.pathX instead of path.x everywhere

Replace path.x to files.pathX in tests

Small fixes to files.js and one rename

Make cleanup run in a fiber

Make wrapping functions take function name in case we need it

Add some timeouts and stuff to HCP tests

wrapFsFunc also makes a sync version of the function

Sometimes you just don't want to yield!

Make sure JsImage readFromDisk doesn't yield

Remove unused imports from npm test

Change order of test now that some things don't yield

Fix missing files import, and add a debug error printout
Add comment explaining existsSync