feat: add deprecation notice for depends_on

[baszalmstra]

This PR reverts the deprecation error on depends_on in favor of a deprecation warning.

We did not really have a way to signal warnings like deprecations in the manifest. This PR adds a simple mechanism to collect warnings during parsing. These warnings are emitted after the project is parsed. Im a little unsure whether it makes sense to just emit them or if we should add a method to emit them.

An example when running pixi list --no-install on the holoviews repository.

 WARN Encountered 2 warnings while parsing the manifest:

  ⚠ The `depends_on` field is deprecated. Use `depends-on` instead.
     ╭─[pixi.toml:136:1]
 135 │ cmd = 'pytest holoviews/tests/ui --ui --browser chromium'
 136 │ depends_on = ["_install-ui"]
     · ─────┬────
     ·      ╰── replace this with 'depends-on'
 137 │
     ╰────


  ⚠ The `depends_on` field is deprecated. Use `depends-on` instead.
     ╭─[pixi.toml:180:1]
 179 │ [feature.doc.tasks.docs-build]
 180 │ depends_on = ['_docs-generate-rst', '_docs-refmanual', '_docs-generate']
     · ─────┬────
     ·      ╰── replace this with 'depends-on'
 181 │
     ╰────

Fixes: #2888

[Hofer-Julian]

Looks good, thanks @baszalmstra!

One small comment, then it can go in from my side.