This replaces the github.com/pborman/uuid dependency with
github.com/google/uuid because the former is only a wrapper for
the latter (since v1.0.0).

Co-authored-by: Felix Lange <fjl@twurst.com>

* accounts/keystore: reduce file name length

* accounts/keystore: reduce code line width

* Fix for #15668

go get github.com/gordonklaus/ineffassign
ineffassign .

In order to avoid disk thrashing for Accounts and HasAccount,
address->key file mappings are now cached in memory. This makes it no
longer necessary to keep the key address in the file name. The address
of each key is derived from file content instead.

There are minor user-visible changes:

- "geth account list" now reports key file paths alongside the address.
- If multiple keys are present for an address, unlocking by address is
  not possible. Users are directed to remove the duplicate files
  instead. Unlocking by index is still possible.
- Key files are overwritten written in place when updating the password.

The account management API was originally implemented as a thin layer
around crypto.KeyStore, on the grounds that several kinds of key stores
would be implemented later on. It turns out that this won't happen so
KeyStore is a superflous abstraction.

In this commit crypto.KeyStore and everything related to it moves to
package accounts and is unexported.