fix(c-api) Rename lib's name to wasmer

[Hywan]

Description

This patch does several things.

1. For the crate wasmer-c-api, the library name is modified from
   wasmer_c_api to wasmer in Cargo.toml. That way, the new
   library files are named libwasmer.* rather than
   libwasmer_c_api.*. That's the primaly goal of this patch. The
   rest is a consequence of this point. Why do we want that? Because
   the build.rs script of the wasmer-c-api crate will configure
   the soname (on Linux), the install_name + current_version +
   compatibility_version (on macOS), and the out-implib +
   output-def (on Windows) for a library named libwasmer, which is
   the name we provide in the Wasmer package for the Wasmer
   libraries. If we want everything to be testable, we cannot use
   libwasmer in soname for a file named libwasmer_c_api for
   example. If we rename the file when packaging (as it's done prior
   this patch), we would need to re-update all those information in
   the Makefile. It implies to duplicate the code in 2 places. So
   let's do things right directly and only once: We want the library
   to be named libwasmer, let's do that.
2. For the crate wasmer-c-api, since the library name has been
   renamed to wasmer, it creates a conflict with the wasmer crate
   which is a dependency. Consequently, this patch also updates the
   Cargo.toml file to modifiy the dependency name with the special
   package attribute (see
   https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#renaming-dependencies-in-cargotoml
   to learn more). So now, the wasmer refers to the wasmer_c_api
   crate, and wasmer-api refers to the wasmer crate.
3. The code of the wasmer-c-api crate is updated accordingly. The
   WasmerEnv derive procedural macro fails because it expects a
   crate named wasmer (which is now wasmer_api), so we implement
   the WasmerEnv trait by hand.
4. The patch updates the build.rs script of the wasmer-c-api
   crate:
   1. In the build_cdylib_link_arg function: The dependency to the
      cdylib-link-lines crate has been removed because the output is
      not exactly the one we expect. So we compute all the
      cargo:rustc-cdylib-link-arg=… lines by hand. The version number
      no longer appears in the library file name for example.
   2. In the build_inline_c_env_vars function: Values passed to
      LDFLAGS have been updated to be libwasmer rather than
      libwasmer_c_api.
   3. A new shared_object_dir helper function has been created
      because it's used in build_inline_c_env_vars and in
      build_cdylib_link_arg.
5. The Makefile has been updated:
   1. In package-capi, we no longer rename libwasmer_c_api to
      libwasmer since the name is correctly defined since the
      beginning now.
      Calling install_name_tool on macOS is no longer required since
      install_name is correctly set by the linker in the build.rs
      script of wasmer-c-api.
   2. In package-docs, some stuffs have been fixed, like the
      SOURCE_VERSION variable that didn't exist, so removed, or the
      mkdir command that was incorrect etc.
   3. In build-docs, the wasmer-c-api crate is excluded from the
      list of crates to generate the documentation for. Mostly because
      the build-docs-capi recipe exists, and we must use it to
      generate the documentation of wasmer-c-api now.
   4. In build-docs-capi, we generate the documentation for the
      wasmer-c-api crate. But rustdoc uses the library name for the
      directory name in the target/doc/ directory. Since the library
      name is now wasmer, it creates a conflict with the wasmer
      crate. Consequently, we update the library name by using sed on
      the Cargo.toml file before running cargo doc, to finally
      restore Cargo.toml as it was previously.

Fixes #2429.

Review

• Add a short description of the change to the CHANGELOG.md file

[Hywan]

bors try

[Hywan]

Closed in favor of #2457.