Use reusable venvs

Replace pip install --target with venvs.
As setuptools breakes out of the venv to load distutils build_ext
command, first create a copy of the Python distribution and hack
it.

This also has the benefit that the venv can be used to build
extensions outside of PyOxidizer.

Also add venv_path to PipRequirementsFile, allowing the same
venv to be incrementally populated in multiple rules,
and the venv re-used across PyOxidizer build runs.

Fixes #162
Fixes #170
Closes #194

pyoxidizer/Cargo.toml

@@ -27,7 +27,9 @@ cc = "1.0"
 clap = "2.32"
 codemap = "0.1"
 codemap-diagnostic = "0.1"
+copy_dir = "0.1.2"
 encoding_rs = "0.8"
+filetime = "0.2"
 git2 = "0.9"
 glob = "0.3"
 goblin = "0.0"

pyoxidizer/src/app_packaging/config.rs

@@ -26,6 +26,7 @@ pub enum InstallLocation {
 #[derive(Clone, Debug, PartialEq)]
 pub struct PackagingSetupPyInstall {
     pub path: String,
+    pub venv_path: Option<String>,
     pub extra_env: HashMap<String, String>,
     pub extra_global_arguments: Vec<String>,
     pub optimize_level: i64,
@@ -87,6 +88,7 @@ pub struct PackagingPackageRoot {
 #[derive(Clone, Debug, PartialEq)]
 pub struct PackagingPipInstallSimple {
     pub package: String,
+    pub venv_path: Option<String>,
     pub extra_env: HashMap<String, String>,
     pub optimize_level: i64,
     pub excludes: Vec<String>,
@@ -99,6 +101,7 @@ pub struct PackagingPipInstallSimple {
 pub struct PackagingPipRequirementsFile {
     // TODO resolve to a PathBuf.
     pub requirements_path: String,
+    pub venv_path: Option<String>,
     pub extra_env: HashMap<String, String>,
     pub optimize_level: i64,
     pub include_source: bool,