variant resolution is overly strict

[trws]

A depends_on() with no variants specified is concretized as requiring a version built with exactly default variants because of the concretize_variants behavior. I would contend that this is unnecessary, and actually unfortunate. This is the root cause of #267 for example, and requires rebuilds to get a less complete version when variants are used to enable extra features. For example, testing the new llvm rework by adding a dependent rust compiler package, depending on llvm required a rebuild of llvm because the installed version had +gold when the default was -gold.

Why are variants this draconian, rather than accepting any package that matches all specified variants?

[trws]

At @tgamblin 's behest I've taken a look at generating a unit test reproducer for this, and actually don't seem to have much luck getting it to show up with spec.satisfies(). That said, it shows up pretty much immediately just on the command line, or with a package. I'm fully willing to admit there may be something I'm doing wrong here, but this is what I'm seeing.

jemalloc+stats does not satisfy jemalloc:

scogland at rzalastor2 in ~/spack (llvm-rework●●)
$ spack install jemalloc+prof+stats
==> Installing jemalloc
==> Trying to fetch from https://github.com/jemalloc/jemalloc/releases/download/4.0.4/jemalloc-4.0.4.tar.bz2
######################################################################## 100.0%
==> Staging archive: /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-de2oo4f3slt2swusob4jkmolrxjupu4e/jemalloc-4.0.4.tar.bz2
==> Created stage in /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-de2oo4f3slt2swusob4jkmolrxjupu4e.
==> No patches needed for jemalloc.
==> Building jemalloc.
==> Successfully installed jemalloc.
  Fetch: 1.66s.  Build: 11.52s.  Total: 13.17s.
[+] /g/g12/scogland/spack/opt/spack/chaos_5_x86_64_ib/gcc-5.3.0/jemalloc-4.0.4-de2oo4f3slt2swusob4jkmolrxjupu4e

scogland at rzalastor2 in ~/spack (llvm-rework●●)
$ spack install jemalloc+prof
==> Installing jemalloc
==> Trying to fetch from https://github.com/jemalloc/jemalloc/releases/download/4.0.4/jemalloc-4.0.4.tar.bz2
######################################################################## 100.0%
==> Staging archive: /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-wl6eoh44fmlwt4himweeu23jjmxn7kap/jemalloc-4.0.4.tar.bz2
==> Created stage in /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-wl6eoh44fmlwt4himweeu23jjmxn7kap.
==> No patches needed for jemalloc.
==> Building jemalloc.
==> Successfully installed jemalloc.
  Fetch: 1.48s.  Build: 11.96s.  Total: 13.44s.
[+] /g/g12/scogland/spack/opt/spack/chaos_5_x86_64_ib/gcc-5.3.0/jemalloc-4.0.4-wl6eoh44fmlwt4himweeu23jjmxn7kap

A prototype rust package, which contains the line depends_on('jemalloc') after the above shows this behavior:

scogland at rzalastor2 in ~/spack (llvm-rework●●)
$ spack find jemalloc
==> 2 installed packages.
-- chaos_5_x86_64_ib / gcc@5.3.0 --------------------------------
jemalloc@4.0.4+prof~stats  jemalloc@4.0.4+prof+stats

scogland at rzalastor2 in ~/spack (llvm-rework●●)
$ spack install rust
==> Installing rust
==> python is already installed in /g/g12/scogland/spack/opt/spack/chaos_5_x86_64_ib/gcc-5.3.0/python-2.7.11-47mkfjgywa5yj5koqrwubjz3lxack7fh.
==> Installing jemalloc
==> Already downloaded /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-hiuikzewyiudf6c74jxvdiynfehgs3dn/jemalloc-4.0.4.tar.bz2.
==> Already staged jemalloc in /g/g12/scogland/spack/var/spack/stage/jemalloc-4.0.4-hiuikzewyiudf6c74jxvdiynfehgs3dn.
==> No patches needed for jemalloc.
==> Building jemalloc.

Neither jemalloc is found and used where "jemalloc" without any variants was specified. This does not seem to conform to the model I think is intended by default.

[mathstuf]

See #839 for an outline of an algorithm to implement this.