Move support for optional features from `rattler` to `resolvo`

[prsabahrami]

After some discussions, @baszalmstra and I realized It is best to move the support for optional features to resolvo's side. Currently the optional features are handled on rattler's side by creating a new solvable per each requested extra with dependency on the original solvable. This causes the conflict graph to grow extensively as there can be different solvables per each extra per each version. An example requesting only one extra from a sample package where multiple package versions provide a certain extra:

Error: Cannot solve the request because of: The following packages are incompatible
├─ foo[with-bar] * cannot be installed because there are no viable options:
│  ├─ foo[with-bar] 3.5.0 would require
│  │  └─ foo ==3.5.0 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│  │     └─ foo 3.5.0, which conflicts with the versions reported above.
│  ├─ foo[with-bar] 3.4.0 would require
│  │  └─ foo ==3.4.0 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│  │     └─ foo 3.4.0, which conflicts with the versions reported above.
│  ├─ foo[with-bar] 3.3.0 would require
│  │  └─ foo ==3.3.0 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│  │     └─ foo 3.3.0, which conflicts with the versions reported above.
│  ├─ foo[with-bar] 3.2.0 would require
│  │  └─ foo ==3.2.0 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│  │     └─ foo 3.2.0, which conflicts with the versions reported above.
│  ├─ foo[with-bar] 3.1.0 would require
│  │  └─ foo ==3.1.0 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│  │     └─ foo 3.1.0, which conflicts with the versions reported above.
│  └─ foo[with-bar] 3.0.2 would require
│     └─ foo ==3.0.2 py36h1af98f8_1[md5=fb731d9290f0bcbf3a054665f33ec94f, sha256=67a63bec3fd3205170eaad532d487595b8aaceb9814d13c6858d7bac3ef24cd4], which cannot be installed because there are no viable options:
│        └─ foo 3.0.2, which conflicts with the versions reported above.
└─ foo >=4.0 cannot be installed because there are no viable options:
   └─ foo 4.0.2, which conflicts with the versions reported above.

Hence, it is best to move the support to resolvo which would be somewhat similar to #101 with the difference that the value of the conditional literals is known in advance.