dapp looks inside ./lib/ even for "innocent" commands

This is quite weird behaviour:
```
~ » mkdir /tmp/dappthing && cd /tmp/dappthing
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/tmp/dappthing » dapp --version
dapp 0.32.2
solc, the solidity compiler commandline interface
Version: 0.6.7+commit.b8d736ae.Linux.g++
hevm 0.46.0
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/tmp/dappthing » mkdir -p lib/foo
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/tmp/dappthing » dapp --version
dapp-remappings: error: ./lib/foo is not a Git repository
dapp-remappings: error: try "dapp update" to initialize submodules
dapp 0.32.2
solc, the solidity compiler commandline interface
Version: 0.6.7+commit.b8d736ae.Linux.g++
hevm 0.46.0
```

Namely, there are some commands, for example `dapp --version`, `dapp --nix-run` etc. that shouldn't expect us to be inside a dapp-structured project. I was told this might be relevant for PR #719