vcpkg export requires packages directory

[katusk]

Describe the bug
The vcpkg export command fails if the packages directory (under the vcpkg root directory) does not contain the to-be-exported package. If you vcpkg install a package with the --clean-after-build switch, the packages directory is not going to be present after a successful install.

Environment

• OS: Windows 10
• Compiler: Microsoft Visual Studio Professional 2017 Version 15.9.16

To Reproduce
Using the flatbuffers library as an example, you can use any other libraries if you want, the results should be the same. Example steps to reproduce the behavior with a x64-windows triplet (the triplet itself is irrelevant):

1. .\vcpkg remove flatbuffers:x64-windows
2. .\vcpkg install flatbuffers:x64-windows --clean-after-build
3. .\vcpkg export flatbuffers:x64-windows --raw --output="vcpkg-export-flatbuffers"
4. See error Source directory C:/workspace/vcpkg/packages/flatbuffers_x64-windows does not exist (assuming your vcpkg root is at C:/workspace/vcpkg).

Results are the same with any of the other export types: "--nuget --ifw --zip --7zip".

Expected behavior
The vcpkg export command in the above should succeed even if the packages directory is not present in the vcpkg root directory. If you omit the --clean-after-build switch (packages directory remains intact), the export command succeeds:

1. .\vcpkg remove flatbuffers:x64-windows
2. .\vcpkg install flatbuffers:x64-windows
3. .\vcpkg export flatbuffers:x64-windows --raw --output="vcpkg-export-flatbuffers"

Failure logs

c:\workspace\vcpkg>vcpkg export flatbuffers:x64-windows --raw --output="vcpkg-export-flatbuffers"
The following packages are already built and will be exported:
    flatbuffers:x64-windows
Exporting package flatbuffers:x64-windows...
Source directory C:/workspace/vcpkg/packages/flatbuffers_x64-windows does not exist

Additional context
I do not think that the packages directory should be necessary to perform a successful export operation. The installed directory in the vcpkg root directory has all the necessary information (assuming x64-windows triplet for the sake of an example):

• The installed/x64-windows directory has all the installed binaries and headers at one place.
• The *_x64-windows.list files under installed/vcpkg/info have all the information describing what files in the installed/x64-windows directory belong to a given library.

E.g. on my machine right now, the installed/vcpkg/info/flatbuffers_1.11.0-1_x64-windows.list file contains the following:

x64-windows/
x64-windows/debug/
x64-windows/debug/lib/
x64-windows/debug/lib/flatbuffers.lib
x64-windows/include/
x64-windows/include/flatbuffers/
x64-windows/include/flatbuffers/base.h
x64-windows/include/flatbuffers/code_generators.h
x64-windows/include/flatbuffers/flatbuffers.h
x64-windows/include/flatbuffers/flatc.h
x64-windows/include/flatbuffers/flexbuffers.h
x64-windows/include/flatbuffers/grpc.h
x64-windows/include/flatbuffers/hash.h
x64-windows/include/flatbuffers/idl.h
x64-windows/include/flatbuffers/minireflect.h
x64-windows/include/flatbuffers/reflection.h
x64-windows/include/flatbuffers/reflection_generated.h
x64-windows/include/flatbuffers/registry.h
x64-windows/include/flatbuffers/stl_emulation.h
x64-windows/include/flatbuffers/util.h
x64-windows/lib/
x64-windows/lib/flatbuffers.lib
x64-windows/share/
x64-windows/share/flatbuffers/
x64-windows/share/flatbuffers/FlatbuffersConfig.cmake
x64-windows/share/flatbuffers/FlatbuffersConfigVersion.cmake
x64-windows/share/flatbuffers/FlatbuffersTargets-debug.cmake
x64-windows/share/flatbuffers/FlatbuffersTargets-release.cmake
x64-windows/share/flatbuffers/FlatbuffersTargets.cmake
x64-windows/share/flatbuffers/FlatcTargets-debug.cmake
x64-windows/share/flatbuffers/FlatcTargets-release.cmake
x64-windows/share/flatbuffers/FlatcTargets.cmake
x64-windows/share/flatbuffers/copyright
x64-windows/share/flatbuffers/vcpkg_abi_info.txt
x64-windows/tools/
x64-windows/tools/flatbuffers/
x64-windows/tools/flatbuffers/flatc.exe

[katusk]

Hmm, both

• https:/microsoft/vcpkg/blob/master/toolsrc/src/vcpkg/export.cpp#L456 and
• https:/microsoft/vcpkg/blob/master/toolsrc/src/vcpkg/commands.exportifw.cpp#L477

call Install::install_files_and_write_listfile (searched for "Exporting package"), that does what its name suggests, and checks for the source directory (searched for "Source directory"; this is substituted for the appropriate subdirectory under packages if I am not mistaken) at

• https:/microsoft/vcpkg/blob/master/toolsrc/src/vcpkg/install.cpp#L55

I think we should call something like read_listfile_and_export_files (just made this name up) at this point and skip checks that are related to the packages directory.

In my mind, at least, the natural sequence of steps for a vcpkg install (points 1-4) and then a vcpkg export (point 5) are the following:

1. Download the package (downloads directory)
2. Build the package (buildtrees directory)
3. Gather build artifacts for install (packages directory)
4. Install the package (installed directory)
5. Export the package (custom export directory)

...where every step should only rely on files from a previous step, I think. And I also consider the directories in points 1-3 to be temporary that could be deleted at any time based on the behaviour and help description of the --clean-after-build switch.

Please let me know what is the best way to proceed, and I am happy to send a PR with the necessary changes.