Auto merge of #5844 - bundler:seg-speed-up-no-op-install, r=colby-swandale

Speed up no-op installs

### What was the end-user problem that led to this PR?

The problem was that a "no-op `bundle install`", defined as a user running `bundle install` when `bundle check` would have succeeded was taking roughly 4 times as long as `bundle check`.

Closes #5842.

### What was your diagnosis of the problem?

My diagnosis was that there were a few different bottlenecks:

- loading plugins
- using `Bundler::Worker` when the work done in that workers block would finish nearly instantly
- some unoptimized code in the parallel installer

### What is your fix for the problem, implemented in this PR?

My fix avoids the bottlenecks by checking if the `install` is guaranteed to be a no-op and short-circuiting all but the core `install` method, which in turn will now avoid the `Bundler::Worker` overhead and quickly cache gems when needed.

### Why did you choose this fix out of the possible options?

I chose this fix because it succeeded in speeding up no-op installs by a _huge_ margin.

This _should_ make `install` nearly as fast as a `bundle check`, but at the moment it's still around ~200ms slower. Not an insignificant gap, but a far cry from what it was before the patch. Additionally, since the `install` code path does things such as ensuring everything is cached.

### Time

Run against https://gist.github.com/indirect/6f60f0db1e2f86804ca3ac7c01f2afc4

```
$ time dbundle check
The Gemfile's dependencies are satisfied

BUNDLE_DISABLE_POSTIT=1 ruby  ~/Development/OpenSource/bundler/exe/bundle   0.32s user 0.06s system 94% cpu 0.403 total
```

```
$ time bundle check
The Gemfile's dependencies are satisfied
BUNDLE_DISABLE_POSTIT=1 ruby  ~/Development/OpenSource/bundler/exe/bundle   0.34s user 0.05s system 96% cpu 0.404 total
```

```
$ time dbundle install --quiet
BUNDLE_DISABLE_POSTIT=1 ruby  ~/Development/OpenSource/bundler/exe/bundle    0.48s user 0.07s system 95% cpu 0.582 total
```

```
$ time bundle install --quiet
BUNDLE_DISABLE_POSTIT=1 ruby  ~/Development/OpenSource/bundler/exe/bundle    1.68s user 0.30s system 92% cpu 2.146 total
```

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