Add source that gets looked up only when source = "<source>" is specified

[chunleng]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the FAQ and general documentation and believe that my question is not already covered.

Feature Request

When dealing with multiple source, some sources are used in only a small portion of the dependency but gets scanned whenever we try to resolve dependency. It will be great to have a option for source such as the following:

[[tool.poetry.source]]
name = "pytorch-cpu"
url = "https://download.pytorch.org/whl/cpu"
source_only = true

Unlike secondary which only decides that the source gets scanned before or after PyPI, this feature will allow the source to never be used as an alternative source

[neersighted]

Duplicate #6713.

[github-actions]

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