Support multiple Nimble dirs

[CyberTailor]

This PR closes issue #956. It is more complex than PR #965 (--destDir) but hypothetically solves wider range of use cases.

Important: only the main Nimble dir can be modified, the others are read-only (this is necessary for packaging in Linux distributions).

Some regressions:

• uninstall --inclDeps can no longer be used with --nimbleDir - use --excludeNimbleDir to leave only one Nimble dir (workaround applied in test suite)
• assert result.metaData.vcsRevision == notSetSha1Hash removed because it no longer works for some reason

Some edge cases are probably not covered.

[CyberTailor]

Should I remove --excludeNimbleDir option and do the following instead: add options.config.nimbleDir only if there are no other Nimble dirs?