Clarify that we build isopackets in order
style: space after bang
simplify compiler.getBuildOrderConstraints

We no longer include versions in the interface. The version was only
used to do some error checking, and the error message in question hasn't
shown up in any bugs from users.

Notably, we were NOT using the version to trigger downloads (that
happens in a different place), so it's not super useful to talk about
multiple versions at all.
eliminate compiler.getBuildOrderConstraints

This gets rid of a call to determineBuildTimeDependencies, which calls
the constraint solver. This is step one of getting rid of
determineBuildTimeDependencies.

Note that this changes the build order logic a little.

In 0.8.3 and earlier we always built *all* the packages we *directly*
depended on before building ourselves. And this led to building the
transitive closure, or at least up until we hit pre-built (release)
packages, which couldn't depend on non-release packages anyway.

In 0.9.0 we only built packages we depend on that have plugins, and the
complete transitive closure of plugins within ourself.  This required
the heavierweight constraint solver call, or at least futzing with the
catalog. I think the "don't build direct dependencies without plugins"
isn't a necessary optimization and it's canceled out by requiring you to
run the constraint solver.  The transitive closure issue is real, but
before merging this branch we'll move the _build logic out of
LocalCatalog.
Remove 'meteor rebuild' and rebuildLocalPackages

This is the first step in removing all references to
PACKAGESOURCE/.build.packagename caches. We will then implement a new
package cache from the ground up.
Stop using the .build.p cache in LocalCatalog
Get rid of a few random spots that refer to .build.
PackageCache: Remove on-disk cache and soft reload

On-disk cache will be replaced by a new class.

Soft refresh was an optimization that wasn't actually very optimized in
0.9.0; it used to only have to look at an in-memory WatchSet and check
those files, but in 0.9.0 it also re-read the whole isopack. Will add
this optimization back in if necessary and safe later.