Extend RepoIndexState syntax to include modifiers

[phadej]

1. ignore revisions (pick first, not last)
2. include releases at given index-timestamp, but include more recent revisions until a second timestamp
3. pin individual packages to specific revision (via sha256, I think we don't want "count")

The 1st and 2nd points are mutulally exclusive, 3rd should be repeated.

Currently we have

hackage.haskell.org,2020-03-22T14:34:00Z

like syntax, i.e. RepoName comma timestamp.

In #6597 I made configuration with colon instead of comma, at least for now. Yet, we need a syntax which works for all of above modifications, is not too convoluted and easy to parse, and perfectly also expandable.

One option is to use spaces.

[phadej]

One rough idea:

With spaces

parser for TotalIndexSpace would similar as now, but requiring
comma separator.

The RepoIndexSpace would be a list of things separated by spaces.

These are compatible with current index-state:

HEAD
2020-03-24T14:34:00Z
@123456789

But we can have more modifiers:

vanilla
revisions-until:2020-03-13T14:34:00Z
revision:package-foobar-1.2.3:b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c

with general rule unqual-component-name like token (: specifiers)?.
Token is required to start with alpha character.
Note that HEAD is such. ISO8601 date and posix timestamps start with digit
and @ so are different.

Examples:

hackage.haskell.org HEAD
hackage.haskell.org 2020-03-24T14:34:00Z
hackage.haskell.org @a23345679
hackage.haskell.org 2019-12-31T23:59:59Z revisions-until:2020-03-24T14:34:00Z
hackage.haskell.org HEAD revision:tree-diff-1.2.3:b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c

That would look like in cabal.project or cabal.project.freeze file like

index-state:
  hackage.haskell.org 
    2019-12-31T23:59:59Z revisions-until:2020-03-24T14:34:00Z
    revision:tree-diff-1.2.3:b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c
    revision:another-pkg-1:7d865e959b2466918c9863afca942d0fb89d7c9ac0c99bafc3749504ded97730

  , head.hackage 2020-03-24T14:34:00Z

Note: eventually when cabal.project parser is remade with parsec, we get leading comma.

With cli you could say

cabal v2-update 'hackage.haskell.org revision:tree-diff-1.2.3:b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c'

or just

cabal v2-update revision:tree-diff-1.2.3:b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c

to pin single revision back.