support the "path" key in Project.toml, (JuliaLang#3756)

code loading has support for this so Pkg should too

src/Types.jl

@@ -247,6 +247,7 @@ Base.@kwdef mutable struct Project
     uuid::Union{UUID, Nothing} = nothing
     version::Union{VersionTypes, Nothing} = nothing
     manifest::Union{String, Nothing} = nothing
+    path::Union{String, Nothing} = nothing
     # Sections
     deps::Dict{String,UUID} = Dict{String,UUID}()
     # deps that are also in weakdeps for backwards compat
@@ -576,7 +577,8 @@ function read_package(path::String)
         pkgerror("expected a `uuid` entry in project file at `$(abspath(path))`")
     end
     name = project.name
-    if !isfile(joinpath(dirname(path), "src", "$name.jl"))
+    pkgpath = joinpath(dirname(path), something(project.path, ""))
+    if !isfile(joinpath(pkgpath, "src", "$name.jl"))
         pkgerror("expected the file `src/$name.jl` to exist for package `$name` at `$(dirname(path))`")
     end
     return project

src/project.jl

@@ -122,6 +122,7 @@ function Project(raw::Dict; file=nothing)
     project.other    = raw
     project.name     = get(raw, "name", nothing)::Union{String, Nothing}
     project.manifest = get(raw, "manifest", nothing)::Union{String, Nothing}
+    project.path     = get(raw, "path", nothing)::Union{String, Nothing}
     project.uuid     = read_project_uuid(get(raw, "uuid", nothing))
     project.version  = read_project_version(get(raw, "version", nothing))
     project.deps     = read_project_deps(get(raw, "deps", nothing), "deps")
@@ -179,6 +180,7 @@ function destructure(project::Project)::Dict
     entry!("uuid",     project.uuid)
     entry!("version",  project.version)
     entry!("manifest", project.manifest)
+    entry!("path",     project.path)
     entry!("deps",     merge(project.deps, project._deps_weak))
     entry!("weakdeps", project.weakdeps)
     entry!("extras",   project.extras)