Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

src/libexpr/value/context.cc

@@ -1,108 +0,0 @@
-#include "util.hh"
-#include "value/context.hh"
-
-#include <optional>
-
-namespace nix {
-
-NixStringContextElem NixStringContextElem::parse(
-    std::string_view s0,
-    const ExperimentalFeatureSettings & xpSettings)
-{
-    std::string_view s = s0;
-
-    std::function<SingleDerivedPath()> parseRest;
-    parseRest = [&]() -> SingleDerivedPath {
-        // Case on whether there is a '!'
-        size_t index = s.find("!");
-        if (index == std::string_view::npos) {
-            return SingleDerivedPath::Opaque {
-                .path = StorePath { s },
-            };
-        } else {
-            std::string output { s.substr(0, index) };
-            // Advance string to parse after the '!'
-            s = s.substr(index + 1);
-            auto drv = make_ref<SingleDerivedPath>(parseRest());
-            drvRequireExperiment(*drv, xpSettings);
-            return SingleDerivedPath::Built {
-                .drvPath = std::move(drv),
-                .output = std::move(output),
-            };
-        }
-    };
-
-    if (s.size() == 0) {
-        throw BadNixStringContextElem(s0,
-            "String context element should never be an empty string");
-    }
-
-    switch (s.at(0)) {
-    case '!': {
-        // Advance string to parse after the '!'
-        s = s.substr(1);
-
-        // Find *second* '!'
-        if (s.find("!") == std::string_view::npos) {
-            throw BadNixStringContextElem(s0,
-                "String content element beginning with '!' should have a second '!'");
-        }
-
-        return std::visit(
-            [&](auto x) -> NixStringContextElem { return std::move(x); },
-            parseRest());
-    }
-    case '=': {
-        return NixStringContextElem::DrvDeep {
-            .drvPath = StorePath { s.substr(1) },
-        };
-    }
-    default: {
-        // Ensure no '!'
-        if (s.find("!") != std::string_view::npos) {
-            throw BadNixStringContextElem(s0,
-                "String content element not beginning with '!' should not have a second '!'");
-        }
-        return std::visit(
-            [&](auto x) -> NixStringContextElem { return std::move(x); },
-            parseRest());
-    }
-    }
-}
-
-std::string NixStringContextElem::to_string() const
-{
-    std::string res;
-
-    std::function<void(const SingleDerivedPath &)> toStringRest;
-    toStringRest = [&](auto & p) {
-        std::visit(overloaded {
-            [&](const SingleDerivedPath::Opaque & o) {
-                res += o.path.to_string();
-            },
-            [&](const SingleDerivedPath::Built & o) {
-                res += o.output;
-                res += '!';
-                toStringRest(*o.drvPath);
-            },
-        }, p.raw());
-    };
-
-    std::visit(overloaded {
-        [&](const NixStringContextElem::Built & b) {
-            res += '!';
-            toStringRest(b);
-        },
-        [&](const NixStringContextElem::Opaque & o) {
-            toStringRest(o);
-        },
-        [&](const NixStringContextElem::DrvDeep & d) {
-            res += '=';
-            res += d.drvPath.to_string();
-        },
-    }, raw);
-
-    return res;
-}
-
-}