Add faithful prefix-path/search-path for car files

[linuskendall]

In this way we could specify our config files without having to put the full path.

Currently:

version: 1 
epoch: 346
data:
  car:
    uri: http://168.119.199.114/346/epoch-346.car
  filecoin:
    enable: false
indexes:
  cid_to_offset_and_size:
    uri: /tank/346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-cid-to-offset-and-size.index
  slot_to_cid:
    uri: /tank/346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-slot-to-cid.index
  sig_to_cid:
    uri:  /tank/346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-sig-to-cid.index
  sig_exists:
    uri: /tank/346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-sig-exists.index
  gsfa:
    uri: /tank/346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-gsfa.indexdir

Alternative:

version: 1 
epoch: 346
data:
  car:
    uri: http://168.119.199.114/346/epoch-346.car
  filecoin:
    enable: false
indexes:
  cid_to_offset_and_size:
    uri: 346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-cid-to-offset-and-size.index
  slot_to_cid:
    uri: 346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-slot-to-cid.index
  sig_to_cid:
    uri:  346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-sig-to-cid.index
  sig_exists:
    uri: 346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-sig-exists.index
  gsfa:
    uri: 346/epoch-346-bafyreiefkwiigddaenieszbyvw7gz4tu7pqoj3obijgs5t5mi6nrril4su-mainnet-gsfa.indexdir

So faithful when looking up a file based URI would:

1. check absolute path
2. check with each of the file-search-path elements prepended to the uri path
3. if none of them are found, throw an error

--file-search-path=/tank:/tank2:/tank3