v2-exec does not work properly without using v2-build first

**Describe the bug**
The `v2-exec` subcommand does not work properly if one has not used `v2-build` first, because it makes assumptions about the layout of the `dist-newstyle` directory. In particular,

* it assumes that a directory `dist-newstyle/tmp` exists; and
* it assumes that there is an initialized package database at `dist-newstyle/packagedb/$compiler`.

**To Reproduce**
Steps to reproduce the behavior:

```
$ cd $(mktemp -d)
$ cabal unpack acme-dont-1.1
$ echo 'packages: acme-dont-1.1' > cabal.project
$ cabal v2-install acme-dont
$ cabal v2-exec ghci
Resolving dependencies...
/tmp/.../dist-newstyle/tmp/environment.-24158: createDirectory: does not exist (No such file or directory)
$ mkdir dist-newstyle/tmp
$ cabal v2-exec ghci
GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help
Loaded package environment from /tmp/.../dist-newstyle/tmp/environment.-24973/.ghc.environment.x86_64-linux-8.6.5
ghc: can't find a package database at /tmp/.../dist-newstyle/packagedb/ghc-8.6.5
$ ghc-pkg init dist-newstyle/packagedb/ghc-8.6.5
$ cabal v2-exec ghci
(finally works)
```

Please use version-prefixed commands (e.g. `v2-build` or `v1-build`) to avoid ambiguity.

**Expected behavior**
I expect the first `cabal v2-exec ghci` to work, ideally even without running `v2-install` first.

**System informataion**
```
$ uname -a
Linux harpy 4.19.0-5-amd64 #1 SMP Debian 4.19.37-5 (2019-06-19) x86_64 GNU/Linux
$ cabal --version
cabal-install version 2.4.1.0
compiled using version 2.4.1.0 of the Cabal library
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5
```

**Additional context**
This came up while trying to replicate my `lambdabot` development process (which is currently based on a sandbox) with `v2-*` tools. Notes:
* `v2-repl` will not help in my actual use case because instead of `ghci` I'd be running `lambdabot` and need an environment setup for `mueval` to find the installed libraries.
* I intend to use `v2-install` rather than `v2-build` because I'll be installing to a project-local store anyway, simulating a sandbox. I believe using `v2-install` makes it easier (compared to `v2-build`) to get all the executables into one place.