Lake toolchain update wrongly claims multiple available toolchains

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

If we have a project which has a lean-toolchain with e.g.:

leanprover/lean4:4.22.0-rc4

and a dependency with (note the v):

leanprover/lean4:v4.22.0-rc4

and we update to the latest versions of these dependencies lake will suggest:

warning: toolchain not updated; multiple toolchain candidates:
  leanprover/lean4:v4.22.0-rc4
    from «doc-gen4»
  leanprover/lean4:4.22.0-rc4
    from MD4Lean

Which is confusing given that the two versions are semantically equivalent.

Context

Updating doc-gen's dependencies

Steps to Reproduce

1. Create a project with some Lean version and add a dependency that has a semantically equivalent but not precisely string equal version.
2. lake update
3. :(

Expected behavior: lake should not complain about multiple candidates

Actual behavior: lake does complain about multiple candidates

Versions

[Output of #version or #eval Lean.versionString]
[OS version, if not using live.lean-lang.org.]

Additional Information

leanprover/lean4:v4.22.0-rc4

Impact

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Start by reproducing the warning with a project whose lean-toolchain and dependency use semantically equivalent versions with and without the v prefix, then run lake update. Trace the toolchain candidate comparison in Lake and verify that equivalent versions are treated as one candidate, with no multiple-candidates warning.