fix: In keybase writeInfo, enforce one name for address

[jefft0]

The Keybase interface was written with the assumption of a one-to-one correspondence between a key's name and address. But this needs to be enforced by the code. PR #2685 updated writeInfo for the case of inserting a new key (address) with the same name as an existing key with a different address. In this case, writeInfo uses the name to look up the existing address and deletes the address entry.

This PR does the same for the other case: Inserting a key with the same address as an existing key, but a new name. In this case, writeInfo uses the address to look up the existing key's name, and deletes the name entry.

This is not a breaking change because none of the production code expects to add a key a second time with the same address but a different name. We update the Keybase doc comments to this effect. However, some of the tests in keybase_test.go make this assumption, so this PR fixes them:

• TestSignVerify creates a key with name n2 and exports its public key. It wants to re-import the public key with name n3 and get the public key to check that it is the same public key as n2. We change this test to re-import the public key into a fresh in-memory key store.
• TestExportImportPubKey creates a key with name "john", exports its public key, then re-imports it with the new name "john-pubkey-only" (but the same address). The current test checks that the key can still be fetched under the old name "john". But this breaks the one-to-one correspondence of key name and address. So the test is changed to confirm that the key with the old name is replaced by the new name.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

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[jefft0]

Removed the review/triage-pending label because core dev thehowl approved.