cmd/go: maintain toolchain for godebug default in go mod tidy

[amirHdev]

When godebug default=go1.x implies a newer minimum toolchain than the
go line alone, go mod tidy should preserve a matching toolchain
directive in go.mod.

Without that, tidy can leave go.mod in a state where an older Go
toolchain fails to load the file instead of following the normal
toolchain-switching path.

This change treats godebug default=go1.x as an implicit minimum
toolchain requirement when computing the main module roots for the
tidy rewrite path. It also adds a regression test covering that case.

Fixes #79153

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Amirhosein Akhlaghpoor:

Patch Set 2:

(1 comment)

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