package manager: write deps in a flat format, eliminating the FQN concept

[mlugg]

Previously, the generated @dependencies module contained dependency metadata specified in a hierarchical manner using fully-qualified names. This exposes issues similar to the old --pkg-begin/--pkg-end options for modules: namely, shared dependencies cannot be reasonably modeled.

The new @depedencies module contains generated code like the following (where strings like "abc123" represent hashes):

pub const root_deps = [_]struct { []const u8, []const u8 }{
    .{ "foo", "abc123" },
};

pub const packages = struct {
    pub const abc123 = struct {
        pub const build_root = "/home/mlugg/.cache/zig/blah/abc123";
        pub const build_zig = @import("abc123");
        pub const deps = [_]struct { []const u8, []const u8 }{
            .{ "bar", "abc123" },
            .{ "name", "ghi789" },
        };
    };
};

Each package contains a build root string, the build.zig import, and a mapping from dependency names to package hashes. There is also such a mapping for the root package dependencies.

In theory, we could now remove the dep_prefix field from std.Build, since its main purpose is now handled differently. I believe this is a desirable goal, as it doesn't really make sense to assign a single FQN to any package (because it may appear in many different places in the package hierarchy). This commit does not remove that field, as it's used non-trivially in a few places in the build runner and compiler tests: this will be a future enhancement.

Resolves: #16354
Resolves: #17135

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Note that this change is difficult to test, as it requires setting up package hierarchies which can be fetched by the package manager. I used a test case posted by @IntegratedQuantum to confirm, to the best of my ability, that it does resolve these issues. I've spoken to @slimsag (as the owner of the biggest project currently affected by these issues): he's happy for this to be merged, after which he can apply the needed changes to Mach's libraries and more definitely confirm that this change functions as desired.