Split Request to PinnableRequest and PinnedRequest

[jrray]

This takes advantage of the work in #1295 in order to guarantee via
types that var requirements have values in the context of a package
requirements.

The PinnableRequest type is what was previously called Request. This
models what can appear in the install.requirements section of a recipe,
as in:

# in a recipe
install:
  requirements:
    - var: foo
      fromBuildEnv: true

When the package is built then a value must have been provided via
options somewhere, and so the var becomes "pinned":

# in a package
install:
  requirements:
    - var: foo/bar

While previously all code that operated on a Request would have to
contend with the type allowing for the value to be missing (or not yet
pinned), now a PinnedRequest is used that guarantees the value is
present.

This change also enforces via the type system that when a recipe is
turned into a package that all the requirements are pinned. A related bug was recently discovered and fixed in #1302.

PinnedRequest did not get a different type for its Pkg variant
(yet?), but it also has a similar pinned/unpinned concept.

[jrray]

This is a good example of the new flow from recipe to package, since they are now different types rendering pins is not an in-place operation.

[jrray]

I found these todo!s still hanging out, but in practice they are unreachable. Now the code path isn't possible.

[codecov]

Codecov Report

❌ Patch coverage is 85.13514% with 132 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ma/crates/foundation/src/ident/pinnable_request.rs	54.38%	26 Missing ⚠️
crates/spk-schema/src/requirements_list.rs	78.40%	19 Missing ⚠️
crates/spk-schema/src/option.rs	84.84%	10 Missing ⚠️
...ates/spk-solve/src/solvers/resolvo/spk_provider.rs	89.89%	10 Missing ⚠️
...hema/crates/foundation/src/ident/pinned_request.rs	93.39%	7 Missing ⚠️
crates/spk-schema/src/v1/platform.rs	50.00%	7 Missing ⚠️
.../crates/foundation/src/spec_ops/has_build_ident.rs	33.33%	6 Missing ⚠️
crates/spk-schema/src/recipe.rs	50.00%	6 Missing ⚠️
...ates/spk-schema/src/v0/recipe_install_spec_test.rs	33.33%	6 Missing ⚠️
crates/spk-schema/src/package.rs	44.44%	5 Missing ⚠️
... and 14 more

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