fix: skip license-file validation when build.source is git or URL

[baszalmstra]

Description

When a package specifies a build source via [package.build.source], the license-file and readme files should be validated against the appropriate directory:

• Git or URL source: skip validation (files are in remote location)
• Path source: validate against the resolved path directory
• No source: validate against the manifest directory (default behavior)

Previously, pixi would always validate these files against the manifest directory, causing false "file does not exist" errors when the files were expected to be in a different location.

Fixes: #4985

An akward side note is that the license can now never be relative to the pixi.toml. Im not sure if thats desired.

How Has This Been Tested?

There are a large number of unit tests that test different edge cases.

I also user tested this by making a new pixi project and point it to another source folder on my disk. The error disappeared.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude Opus 4.5

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

While testing this it now looks like the LICENSE cannot be found at all during the build when using an out of source build. I think the reason is that we do not pass the full path to the license to the backend. Will need some more investigation.