Split selector logic into conda_build.selectors

[kenodegard]

Description

While tinkering with #5495 I noticed a few local imports to avoid circular imports between conda_build.metadata and conda_build.variants. This is an artifact of how selectors evolved where originally they were purely a meta.yaml feature (hence why it was defined within conda_build.metadata and over time were introduced for the conda_build_config.yaml file.

To better reflect the agnostic nature of selectors I propose moving the related logic into a new submodule:

old path	new path
conda_build.metadata.ARCH_MAP	conda_build.utils.ARCH_MAP
conda_build.metadata.get_selectors	conda_build.selectors.get_selectors
conda_build.metadata.ns_cfg	conda_build.selectors.get_selectors
conda_build.metadata.sel_pat	conda_build.selectors.RE_SELECTOR
conda_build.metadata.parseNameNotFound	conda_build.selectors.parse_NameError
conda_build.metadata.eval_selector	conda_build.selectors.eval_selector
conda_build.metadata._split_line_selector	conda_build.selectors._split_line_selector
conda_build.metadata.select_lines	conda_build.selectors.select_lines

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?