Add an option to export build time dependencies as run_constrained keys and pin strictly

[183amir]

Following the discussion here: #2001 (comment), I am proposing to add the following feature to conda build.
Currently, there is an option in conda build recipes which allows you to pin all the packages that were used during build time as runtime dependencies. See original pull request here: #741

For example a recipe like:

package:
  name: bob-devel

build:
  pin_depends: strict

requirements:
  run:
    - curl 7.52.1

will have the following index.json

{
  "depends": [
    "curl 7.52.1 0",
    "openssl 1.0.2l 0",
    "zlib 1.2.8 3",
  ],
  "name": "bob-devel",
}

where openssl and zlib are dependencies of curl. Note that everything is pinned strictly and normally dependencies of zlib and openssl get pinned strictly too but I did not put it in the example above.

I propose a very similar functionality to be added for run_constrained keys as well.
For example for the following recipe:

package:
  name: bob-devel

build:
  pin_constrains: strict

run_constrained:
  - curl 7.52.1

we get this:

{
  "constrains": [
    "curl 7.52.1 0",
    "openssl 1.0.2l 0",
    "zlib 1.2.8 3",
  ],
  "name": "bob-devel",
}

[183amir]

I was thinking maybe instead of this, we can have a jinja function to help me do this. Something like:

package:
  name: bob-devel
  version: 2018.02.21

requirements:
  host:
    - curl 7.55.1
  run_constrained:
  {% for package in packages('host', all=True) %}
    - {{ pin_compatible(package, exact=True) }}
  {% endfor %}

Would render to:

package:
    name: bob-devel
    version: 2018.02.21
requirements:
    host:
        - ca-certificates 2017.08.26 h1d4fec5_0
        - libgcc-ng 7.2.0 h7cc24e2_2
        - openssl 1.0.2n hb7f436b_0
        - curl 7.55.1 h78862de_4
        - libssh2 1.8.0 h9cfc8f7_4
        - zlib 1.2.11 ha838bed_2
    run_constrained:
        - ca-certificates 2017.08.26 h1d4fec5_0
        - libgcc-ng 7.2.0 h7cc24e2_2
        - openssl 1.0.2n hb7f436b_0
        - curl 7.55.1 h78862de_4
        - libssh2 1.8.0 h9cfc8f7_4
        - zlib 1.2.11 ha838bed_2

[msarahan]

For that, you'd need to add a new jinja2 function to the context. See these two bits to see how pin_compatible works:

https://github.com/conda/conda-build/blob/master/conda_build/jinja_context.py#L192
https://github.com/conda/conda-build/blob/master/conda_build/jinja_context.py#L454

If you put up a PR, I can help on it. Otherwise, this can go in the backlog, and I'm not sure when I'll get to it.

[github-actions]

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