odb: refactor parsing of alternates to be self-contained

Parsing of the alternates file and environment variable is currently
split up across multiple different functions and is entangled with
`link_alt_odb_entries()`, which is responsible for linking the parsed
object database sources. This results in two downsides:

  - We have mutual recursion between parsing alternates and linking them
    into the object database. This is because we also parse alternates
    that the newly added sources may have.

  - We mix up the actual logic to parse the data and to link them into
    place.

Refactor the logic so that parsing of the alternates file is entirely
self-contained. Note that this doesn't yet fix the above two issues, but
it is a necessary step to get there.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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