Fix multiple prefixes in multipass

[strarsis]

imagemin-svgo enables svgo multipass by default, hence many users reports this issue (#1148).

This PR will add a flag to the internal svgo AST for each node processed by the prefixIds plugin to prevent multiple prefixing, which currently happens in multipass mode.

(Originally the prefixIds plugin should check the beginning of each name and skip if it equals the prefix, but there can be occasions where the prefix is the same as a non-prefixed name, and this can result in collisions between SVGs because of skipped prefixing.)

[strarsis]

Retracting this PR in favour of a better solution that adds multipassCount to
the info object that is passed to each plugin: #1173