This change allows the tool to *read* .meteor/versions whether it uses
"@" or " " as a package/version separator. It does not change how
.meteor/versions is written out.
Background:
The convention in the new Version Solver is that "foo@1.2.3" means a
constraint, which is satisfied by "foo 1.2.4" but not "foo 1.2.2", which
are specific versions of a package. This convention avoids the same
confusion of .meteor/packages being a list of constraints while
.meteor/versions is conceptually a list of (package,version) pairs, even
though both look like package@version.
We'd like to replace "@" by " " in .meteor/versions from now on.
Unfortunately, even if the new tool can read old .meteor/versions files,
the old tool wouldn't be able to read the new style. If in some release
of Meteor, we start writing .meteor/versions that use spaces, you won't
be able to switch between newer and older releases of Meteor without
getting a hard error about a malformed versions file in your app.
What we can do, at least, is start letting the tool read the better
format now, and maybe in the future we can switch.
This change also makes parsePackageAtVersion stand alone, instead of
being defined in terms of parsePackageConstraint, which is nice.
4106e8cade