Overly permissive check for mercurial in meta.yaml files

[dpryan79]

This seems to be related to #1488. It turns out that if hg_ (in any case) is present in a meta.yaml file that conda build will assume that mercurial is being used. That's normally not an issue, but when things like linting are being done (at least on bioconda with bioconda-utils lint) we then receive the following error:

Traceback (most recent call last):
  File "/home/dpryan/miniconda3/bin/bioconda-utils", line 11, in <module>
    sys.exit(main())
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/bioconda_utils/cli.py", line 657, in main
    bioconductor_skeleton, pypi_check, clean_cran_skeleton,
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/argh/dispatching.py", line 328, in dispatch_commands
    dispatch(parser, *args, **kwargs)
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/argh/dispatching.py", line 174, in dispatch
    for line in lines:
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/argh/dispatching.py", line 277, in _execute_command
    for line in result:
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/argh/dispatching.py", line 260, in _call
    result = function(*positional, **keywords)
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/bioconda_utils/cli.py", line 243, in lint
    report = linting.lint(_recipes, lint_args)
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/bioconda_utils/linting.py", line 204, in lint
    metas.extend(utils.load_all_meta(recipe, config=config, finalize=False))
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/bioconda_utils/utils.py", line 172, in load_all_meta
    bypass_env_check=bypass_env_check,
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/conda_build/api.py", line 44, in render
    permit_unsatisfiable_variants=permit_unsatisfiable_variants)
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/conda_build/render.py", line 745, in render_recipe
    try_download(m, no_download_source=no_download_source)
  File "/home/dpryan/miniconda3/lib/python3.6/site-packages/conda_build/render.py", line 580, in try_download
    raise ValueError("no_download_source specified, but can't fully render recipe without"
ValueError: no_download_source specified, but can't fully render recipe without downloading source.  Please fix the recipe, or don't use no_download_source.

The cause of that appears to be https://github.com/conda/conda-build/blob/master/conda_build/metadata.py#L1575 , where the whole file is checked for a simple regex (presumably only the source section should be checked).

It's quite simple for me to check for and remove hg_ in the bioconda skeleton code to avoid this, but I figured this should be pointed out so others running into the aforementioned error can find the cause easily.

Below is a meta.yaml file that will reproduce this issue (I think the summary is the issue):

{% set version = "2.18.0" %}
{% set name = "hgu95av2cdf" %}
{% set bioc = "3.8" %}

package:
  name: 'bioconductor-{{ name|lower }}'
  version: '{{ version }}'
source:
  url:
    - 'https://bioconductor.org/packages/{{ bioc }}/data/annotation/src/contrib/{{ name }}_{{ version }}.tar.gz'
    - 'https://bioarchive.galaxyproject.org/{{ name }}_{{ version }}.tar.gz'
    - 'https://depot.galaxyproject.org/software/bioconductor-{{ name }}/bioconductor-{{ name }}_{{ version }}_src_all.tar.gz'
  md5: 026bae2f76433593f75ca6c79bc9a9d0
build:
  number: 0
  rpaths:
    - lib/R/lib/
    - lib/
requirements:
  host:
    - 'bioconductor-annotationdbi >=1.44.0,<1.46.0'
    - r-base
  run:
    - 'bioconductor-annotationdbi >=1.44.0,<1.46.0'
    - r-base
    - wget
test:
  commands:
    - '$R -e "library(''{{ name }}'')"'
about:
  home: 'https://bioconductor.org/packages/{{ bioc }}/data/annotation/html/{{ name }}.html'
  license: LGPL
  summary: 'A package containing an environment representing the HG_U95Av2.CDF file.'

Xref: bioconda/bioconda-utils#353

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