feat: make build plans for snaps with platforms

[lengau]

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Adds a function to make build plans for snaps that use the platforms keyword.

Fixes #5
Requires #39
CRAFT-3008

[mr-cal]

Looking good! I found some missing corner cases for some of snapcraft's esoteric base/build-base definitions

[mr-cal]

Minor feedback on type: snapd and the docs but it looks good! Nice job catching the complexities of base/build-base combinations.

[mr-cal]

One more thought - base: devel should be an error, I don't think it is:

def test_base_devel_error():
    """'base: devel' raises an error."""
    build_plan = _build.get_platforms_snap_build_plan(
        base="devel",
        platforms={"amd64": None},
        build_base=None,
        snap_type="app",
    )

[lengau]

@mr-cal I've added the devel error test - it simply calls the base unknown or invalid.

[lengau]

Re-requesting reviews from @mr-cal and @cmatsuoka because I've reworked this following our discussion

[mr-cal]

Looks great!

[mr-cal]

Note to self: With this latest design, we are moving some of the validation to snapcraft because craft-platforms will accept arbitrary base names.

For example, snapcraft is responsible for raising an error for base: devel or build-base: almalinux@9.

[cmatsuoka]

Nice, this seems to follow all the rules established for snap bases and build-bases.

[cmatsuoka]

Can you add a PR description?