Installed Composer version lacks public keys

[scy]

To reproduce:

1. Install Composer using TCE.
2. Run composer diagnose.

The output will contain these lines:

Checking pubkeys: FAIL
Missing pubkey for tags verification
Missing pubkey for dev verification
Run composer self-update --update-keys to set them up

Running composer self-update --update-keys expects the user to manually paste the keys from https://composer.github.io/pubkeys.html, which is an inconvenience. Also, at least the "tags" key is already included in TCE.

Suggestion: We should find out where Composer expects these keys to be and make them available automatically; at least the "tags" key.

[scy]

On my machine, they can be found at ~/.composer/keys.{dev,tags}.pub, and the tags file contains exactly the contents of TCE's $signature_key string. So, adding the dev key to TCE (or downloading it?) and putting the files in the right places should be sufficient.