fix(manager/pep621): support array form of tool.uv.sources

[ntsk]

Changes

Reported in #44948

uv allows tool.uv.sources entries to be written as an array of sources, disambiguated by environment markers. A single-element array is valid too.
The pep621 schema only accepted the single-table form, and LooseRecord silently drops entries it can't parse, so array-form sources were ignored and lookups fell back to pypi.org.
For packages that only exist on the declared index this shows up as WARN: Package lookup failures.
For packages that also exist on pypi.org the problem is harder to notice: there is no warning, and Renovate silently proposes versions from the wrong registry.

What this PR does:

• schema: sources now accepts a single source or an array of sources, normalized to an array
• processor: when all sources of a dependency reference an index, all resolved index URLs are set as registryUrls (the pypi datasource merges releases across registries). A single non-index source behaves exactly as before.
• multiple sources that are not all indexes (e.g. a git source per platform) get skipReason: 'unsupported'. Renovate can't represent them as a single update, and skipping is still better than the previous behavior of silently looking up pypi.org.

One thing this PR does not attempt: when markers don't cover every environment, uv falls back to the default index for the rest. Renovate doesn't evaluate markers, and the existing single-table handling has the same simplification, so I kept that behavior.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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The public repository: https://github.com/ntsk/reproduce-array-form-uv-sources

The repo pins a package that only exists on test.pypi.org to a custom index using the array form.

DEBUG: GET https://pypi.org/pypi/ntsk-reproduce-array-form-uv-sources/json = (code=ERR_NON_2XX_3XX_RESPONSE, statusCode=404 ...)
DEBUG: Failed to look up pypi package ntsk-reproduce-array-form-uv-sources: no-result

With this branch (node lib/renovate.ts --platform=local in that repo):

"registryUrls": ["https://test.pypi.org/simple/"]

and the lookup failure is gone.