add requires api checker

[Shiffted]

#7378

Adds checker for add_requires() like discussed in #7394

This does not check configs from package deps, e.g.:

package("foo")
    add_deps("libsdl2", {configs = {invalid_conf = true}})
    on_install(function (package) end)
package_end()

add_requires("foo")

will not be checked.

[gemini-code-assist]

Code Review

This pull request introduces new API checkers (api.requires.package and api.requires.configs) to validate package names and configurations in add_requires and add_requireconfs, along with corresponding tests and refactoring of package loading logic. The reviewer identified several issues, including a potential side-effect where table.join2 could mutate the project's internal state in-place, missing defensive checks for opt and requireinfo that could cause runtime errors, and an issue where requireconf is skipped entirely if its source information is missing.

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[waruqi]

To minimize changes, I don't have time to review large patches.

[Shiffted]

I changed the implementation to minimize changes and added a checker only for add_requires(). I will add the add_requireconfs() checker in a separate PR.

This now also allows values checker to skip instances. The only other previously existing values checker always returns a non-nil table, so existing checkers are unaffected.

---

我修改了实现方案，以尽量减少改动，并且仅为 add_requires() 添加了检查器。我将在另一个 PR 中添加 add_requireconfs() 的检查器。

现在，values 检查器也可以跳过实例了。此前唯一存在的另一个 values 检查器始终返回一个非空表，因此现有的检查器不受影响。