Fix resolution of a name that's a prefix of another name

[native-api]

…name

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Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• My PR addresses the following pyenv issue (if any)
  • Closes pyenv-latest may report the wrong prefix #2519

Description

• Here are some details about my PR

Makes a complete name resolve to itself before trying to apply it as a prefix.

I hesitated to add this initially because subcommands that use pyenv-latest already do that -- if the prefix resolution fails, they fall back to using the original name. However, if there are 2 names like foo and foo.bar, pyenv latest foo undesirably succeeds.

Tests

• My PR adds the following unit tests (if any)
  Test for the new use case