[dub-reference/dependencies] Basic contents

[WebFreak001]

• link to recipes page for full syntax
• give thorough examples of all different dependency syntaxes
  • just a version
    • "*" version
    • ~>a.b.c
    • ~>a.b
    • >=a, >=a <b, <b
    • ~gitbranch (discouraged because of it not updating)
  • version + optional (also link to effects on Have_ version identifiers)
  • version + optional + default=false
  • path-based dependency
    • note how this is best used in sub-packages, avoid going outside of repository bounds for public projects
  • git repository based
  • show how dflags (and in the future other build options) can be applied to dependencies
• tell about how dub.selections.json affects them, link to DUB Guide/selections
• tell about how dub add-path affects them
• tell about how dub add-override affects them
• tell about how dub add-local affects them
• (one of these three above here is going to be obsolete, need to search through the dub github again to check which one, or ask @Geod24)
• link to repositories page, mention how maven can be used as format and registry
• describe how dependency resolution / version resolution takes place (dub upgrade, dub build with missing dependencies)
• mention that currently per-configuration dependencies are not supported / are always fetched, even if not using the configuration, because of the selections file (which will require new syntax)
• mention how Have_xyz naming works

See also: #55

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@Geod24

Overrides are deprecated.

I also would not document ~branch, we want to get rid of it, it's better to have support in the git repository anyway.