Change RepoPaths to be acquired via RepoPathCache (#3284)HEADmaster

### **PR Description**
In order to optimize the number of git calls made this change implements
a RepoPathCache as the API by which RepoPaths instances are acquired.
This primarily affects app startup and worktree enumeration.

This introduces a new dependency on
[go-memoize](https://github.com/kofalt/go-memoize), which is a
lightweight wrapper around go-cache and singleflight, in order to ensure
that the cache is concurrency safe. (As compared to a simple map, e.g.)
See the go-memoize README for details.

Fixes #3227.

### **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

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